Laser diode with an improved multiple quantum well structure adopted for reduction in wavelength chirping

ABSTRACT

The present invention provides another active layer structure provided in a light emission device for emitting a light with a predetermined wavelength. The active layer structure comprises a multiple quantum well structure and at least a second well layer. The multiple quantum well structure comprises alternating laminations of first well layers showing electroluminescence and potential barrier layers. The first well layers have a first set of energy band gaps which are uniform and corresponds to the predetermined wavelength, provided that energy band gap is defined as a difference between a ground level of electrons in conduction band and a ground level of holes in valence band. The second well layer is provided within any of the potential barer layers so that the second well layer is separated via the potential barrier layers from the first well layers. The second well layer has a second energy band gap in a range which is above the first set of energy band gaps and below a set of forbidden band widths of the potential barrier layers. The range of the second energy band gaps is defined so that the second well layer exhibits carrier accumulations and no electro-luminescence to thereby ensure that carriers accumulated in the second well layer are injected into the first well layers when the first well layers are deficient in carriers for the electro-luminescence.

This application is a divisional of application Ser. No. 08/625,345,filed Apr. 1, 1996, and issued as U.S. Pat. No. 5,790,578 on Aug. 4,1998.

BACKGROUND OF THE INVENTION

The present invention provides a laser diode within a multiple quantumwell structure.

A conventional laser diode will be described with reference to FIGS. 1and 2A through 2C. The conventional laser diode is provided asillustrated in FIGS. 1 and 2A. The conventional laser diode is formed onan n-type InP substrate 1. A surface of the n-type InP substrate 1 isformed with a grating 9. An optical guide layer 2 is provided whichextends over the grating 9. The optical guide layer 2 is made of InGaAswhich has a wavelength composition of 1.2 micrometers. An active layeris provided, which extends over the optical guide layer 2. The activelayer has a multiple quantum well structure. An optical guide layer 4 isprovided, which extends over the active layer 3. A p-type InP claddinglayer 5 is provided, which extends over the optical guide layer 4. Ap-type InP layer 6 is provided which extends over the p-type InPcladding layer 5. A p-type InGaAsP contact layer 7 is provided whichextends over the p-type InP layer 6. A p-type electrode is provided onthe p-type InGaAsP contact layer 7.

The following descriptions will focus on the multiple quantum wellstructure of the active layer 3 with reference to FIGS. 2B and 2C. Themultiple quantum well structure comprises alternating laminations ofelectroluminescence well layers 31 and potential barrier layers 32.

The electro-luminescence (also spelled "electroluminescence" below )well layer 31 comprises a +0.8%-strained InGaAs layer which has athickness of 5.5 nanometers and a wavelength composition of 1.72micrometers. The potential barrier layer 32 comprises a non-strainedInGaAs which has a thickness of 4.0 micrometers and a wavelengthcomposition of 1.15 micrometers. The electro-luminescence well layer 31has an energy band gap E_(g1) of 0.80 eV, where energy band gap isdefined as a difference between a ground level of electrons inconduction band and a ground level of holes in valence band. The energyband gap E_(g1) of 0.80 eV of the electro-luminescence well layer 31 isconverted into a wavelength of 1.55 micrometers.

The above electro-luminescence well layer 35 having the small energyband gap E_(g1) of 0.80 eV shows electro-luminescence at a wavelength of1.55 micrometers. This electro-luminescence is caused by the inducedtransition by electron-hole recombination. This means that theelectro-luminescence well layers become necessarily deficient incarriers. This makes the electro-luminescence well layers be deficientin carriers for the electro-luminescence. This causes a considerablevariation in refractive index of a laser medium. This enlarges an activewavelength chirping.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide animproved active layer structure provided in a light emission device freefrom the above problems or disadvantages.

It is a further object of the present invention to provide an improvedactive layer structure provided in a light emission device, whichsuppresses an active wavelength chirping.

It is a furthermore object of the present invention to provide animproved active layer structure provided in a light emission device,which causes either almost no variation in refractive index of a lasermedium or a considerable reduction in variation in refractive index of alaser medium.

It is a still further object of the present invention to provide asemiconductor laser device having an improved active layer structurefree from the above problems or disadvantages.

It is yet a further object of the present invention to provide asemiconductor laser device having an improved active layer structurewhich suppresses an active wavelength chirping.

It is moreover object of the present invention to provide asemiconductor laser device having an improved active layer structurewhich causes either almost no variation in refractive index of a lasermedium or a considerable reduction in variation in refractive index of alaser medium.

The above and other objects, features and advantages of the presentinvention will be apparent from the following descriptions.

The present invention provides an active layer structure provided in alight emission device for emitting a light with a predeterminedwavelength. The active layer structure comprises an electroluminescencesection and a carrier accumulation section in alignment with theelectro-luminescence section. The electroluminescence section comprisesa first multiple quantum well structure comprising alternatinglaminations of first well layers showing electroluminescence and firstpotential barrier layers. The first well layers have a first set of auniform energy band gap which corresponds to the predeterminedwavelength. The carrier accumulation section comprises a second multiplequantum well structure comprising alternating laminations of second welllayers and second potential barrier layers, provided that energy bandgap is defined as a difference between a ground level of electrons inconduction band and a ground level of holes in valence band. The secondwell layers have a second set of energy band gaps in a range which isabove the first set of energy band gaps and below a set of forbiddenband widths of the first potential barn-er layers. The range of thesecond set of energy band gaps is defined so that the second well layersexhibit carrier accumulations and no electro-luminescence to therebyensure that carriers accumulated in the second well layers are injectedinto the first well layers when the first well layers are deficient incarriers for the electro-luminescence.

The present invention also provides a semiconductor laser deviceincluding an active layer structure which is provided on an opticalguide layer provided on a surface, having a grating structure, of asemiconductor substrate. The active layer structure comprising anelectroluminescence section and a carrier accumulation section inalignment with the electroluminescence section. The electroluminescencesection comprises a first multiple quantum well structure comprisingalternating laminations of first well layers showing electroluminescenceand first potential barrier layers. The first well layers have a firstset of a uniform energy band gap which corresponds to the predeterminedwavelength. The carrier accumulation section comprises a second multiplequantum well structure comprising alternating laminations of second welllayers and second potential barrier layers, provided that energy bandgap is defined as a difference between a ground level of electrons inconduction band and a ground level of holes in valence band. The secondwell layers have a second set of energy band gaps in a range which isabove the first set of energy band gaps and below a set of forbiddenband widths of the first potential barrier layers. The range of thesecond set of energy band gaps is defined so that the second well layersexhibit carrier accumulations and no electro-luminescence to therebyensure that carriers accumulated in the second well layers are injectedinto the first well layers when the first well layers are deficient incarriers for the electroluminescence.

The present invention provides another active layer structure providedin a light emission device for emitting a light with a predeterminedwavelength. The active layer structure comprises a multiple quantum wellstructure and at least a second well layer. The multiple quantum wellstructure comprises alternating laminations of first well layers showingelectroluminescence and potential barrier layers. The first well layershave a first set of energy band gaps which are uniform and correspondsto the predetermined wavelength, provided that energy band gap isdefined as a difference between a ground level of electrons inconduction band and a ground level of holes in valence band. The secondwell layer is provided within any of the potential barrier layers sothat the second well layer is separated via the potential barrier layersfrom the first well layers. The second well layer has a second energyband gap in a range which is above the first set of energy band gaps andbelow a set of forbidden band widths of the potential barrier layers.The range of the second energy band gaps is defined so that the secondwell layer exhibits carrier accumulations and no electro-luminescence tothereby ensure that carriers accumulated in the second well layer areinjected into the first well layers when the first well layers aredeficient in carriers for the electro-luminescence.

The present invention also provides another semiconductor laser deviceincluding an active layer structure which is provided on an opticalguide layer provided on a surface, having a grating structure, of asemiconductor substrate. The active layer structure comprises a multiplequantum well structure and a multiple quantum well structure. Themultiple quantum well structure comprises alternating laminations offirst well layers showing electroluminescence and potential barrierlayers. The first well layers have a first set of energy band gaps whichare uniform and corresponds to the predetermined wavelength, providedthat energy band gap is defined as a difference between a ground levelof electrons in conduction band and a ground level of holes in valenceband. The multiple quantum well structure is provided within any of thepotential barrier layers so that the second well layer is separated viathe potential barrier layers from the first well layers. The second welllayer has a second energy band gap in a range which is above the firstset of energy band gaps and below a set of forbidden band widths of thepotential barrier layers. The range of the second energy band gaps isdefined so that the second well layer exhibits carrier accumulations andno electro-luminescence to thereby ensure that carriers accumulated inthe second well layer are injected into the first well layers when thefirst well layers are deficient in carriers for the electroluminescence.

The present invention provides still another active layer structureprovided in a light emission device for emitting a light with apredetermined wavelength. The active layer structure comprises amultiple quantum well structure and at least a second well layer. Themultiple quantum well structure comprises alternating laminations offirst well layers showing electroluminescence and potential barrierlayers. The first well layers have a first set of energy band gaps whichare uniform and corresponds to the predetermined wavelength, providedthat energy band gap is defined as a difference between a ground levelof electrons in conduction band and a ground level of holes in valenceband. The second well layer is provided on any interface of the firstwell layers and the potential barrier layers so that the second welllayer is sandwiched between the first well layer and the potentialbarrier layer. The second well layer has a second energy band gap in arange which is above the first set of energy band gaps and below a setof forbidden band widths of the potential barrier layers. The range ofthe second energy band gap is defined so that the second well layerexhibits carrier accumulations and no electro-luminescence. This ensuresthat carriers accumulated in the second well layer are injected into thefirst well layers when the first well layers are deficient in carriersfor the electroluminescence.

The present invention also provides still another semiconductor laserdevice including an active layer structure which is provided on anoptical guide layer provided on a surface, having a grating structure,of a semiconductor substrate. The active layer structure comprises amultiple quantum well structure and at least a second well layer. Themultiple quantum well structure comprises alternating laminations offirst well layers showing electroluminescence and potential barrierlayers. The first well layers have a first set of energy band gaps whichare uniform and corresponds to the predetermined wavelength, providedthat energy band gap is defined as a difference between a ground levelof electrons in conduction band and a ground level of holes in valenceband. The second well layer is provided on any interface of the firstwell layers and the potential barrier layers so that the second welllayer is sandwiched between the first well layer and the potentialbarrier layer. The second well layer has a second energy band gap in arange which is above the first set of energy band gaps and below a setof forbidden band widths of the potential barrier layers. The range ofthe second energy band gap is defined so that the second well layerexhibits carrier accumulations and no electro-luminescence. This ensuresthat carriers accumulated in the second well layer are injected into thefirst well layers when the first well layers are deficient in carriersfor the electroluminescence.

The present invention also provides yet another active layer structureprovided in a light emission device for emitting a light with apredetermined wavelength. The active layer structure comprises amultiple quantum well structure and at least a second well layer. Themultiple quantum well structure comprises alternating laminations offirst well layers showing electroluminescence and potential barrierlayers. The first well layers have a first set of energy band gaps whichare uniform and corresponds to the predetermined wavelength, providedthat energy band gap is defined as a difference between a ground levelof electrons in conduction band and a ground level of holes in valenceband. The second well layer is provided at any side of the multiplequantum well structure so that the second well layer is separated viathe potential barrier layers from the first well layers. The second welllayer has a second energy band gap in a range which is above the firstset of energy band gaps and below a set of forbidden band widths of thepotential barrier layers. The range of the second energy band gap isdefined so that the second well layer exhibits carrier accumulations andno electro-luminescence. This ensures that carriers accumulated in thesecond well layer are injected into the first well layers when the firstwell layers are deficient in carriers for the electro-luminescence.

The present invention also provides yet another semiconductor laserdevice including an active layer structure which is provided on anoptical guide layer provided on a surface, having a grating structure,of a semiconductor substrate. The active layer structure comprises amultiple quantum well structure and at least a second well layer. Themultiple quantum well structure comprises alternating laminations offirst well layers showing electroluminescence and potential barrierlayers. The first well layers have a first set of energy band gaps whichare uniform and corresponds to the predetermined wavelength, providedthat energy band gap is defined as a difference between a ground levelof electrons in conduction band and a ground level of holes in valenceband. The second well layer is provided at any side of the multiplequantum well structure so that the second well layer is separated viathe potential barrier layers from the first well layers. The second welllayer has a second energy band gap in a range which is above the firstset of energy band gaps and below a set of forbidden band widths of thepotential barrier layers. The range of the second energy band gap isdefined so that the second well layer exhibits carrier accumulations andno electro-luminescence. This ensures that carriers accumulated in thesecond well layer are injected into the first well layers when the firstwell layers are deficient in carriers for the electroluminescence.

BRIEF DESCRIPTIONS OF THE DRAWINGS

Preferred embodiments of the present invention will be described indetail with reference to the accompanying drawings.

FIG. 1 is a squint view illustrative of the conventional laser diode.

FIG. 2A is a cross sectional elevation view illustrative of theconventional laser diode of FIG. 1.

FIG. 2B is a diagram illustrative of an energy band gap profile of theconventional multiple quantum well structure of an active layer in theconventional laser diode of FIG. 2A.

FIG. 2C is a diagram illustrative of an energy band gap level of welllayers in the conventional multiple quantum well structure of FIG. 2B.

FIG. 3 is a diagram illustrative of an active wavelength chirpingproperty of the conventional laser diode with the normal multiplequantum well structure.

FIG. 4 a squint view illustrative of a novel laser diode in a firstembodiment according to the present invention.

FIG. 5A is a cross sectional elevation view illustrative of a novellaser diode of FIG. 4.

FIG. 5B is a diagram illustrative of an energy band gap profile of amultiple quantum well structure in an electroluminescence region of anactive layer in a novel laser diode of FIG. 5A.

FIG. 5C is a diagram illustrative of an energy band gap profile of amultiple quantum well structure in a carrier accumulation region of anactive layer in a novel laser diode of FIG. 5A.

FIG. 5D is a diagram illustrative of energy band gap levels of amultiple quantum well structure over an electroluminescence region and acarrier accumulation region in a novel laser diode of FIG. 5A.

FIGS. 6A through 6C are fragmentary cross sectional elevation viewsillustrative of fabrication processes for a novel laser diode of FIG. 4.

FIG. 7 a squint view illustrative of a novel laser diode in a secondembodiment according to the present invention.

FIG. 8A is a cross sectional elevation view illustrative of a novellaser diode of FIG. 7.

FIG. 8B is a diagram illustrative of an energy band gap profile of amultiple quantum well structure of an active layer in a novel laserdiode of FIG. 8A.

FIG. 8C is a diagram illustrative of energy band gap levels of amultiple quantum well structure in a novel laser diode of FIG. 8A.

FIG. 9 a squint view illustrative of a novel laser diode in a thirdembodiment according to the present invention.

FIG. 10A is a cross sectional elevation view illustrative of a novellaser diode of FIG. 9.

FIG. 10B is a diagram illustrative of an energy band gap profile rfmultiple quantum well structure of an active layer in a novel laserdiode of FIG. 10A.

FIG. 10C is a diagram illustrative of energy band gap levels of amultiple quantum well structure in a novel laser diode of FIG. 10A.

FIG. 11 a squint view illustrative of a novel laser diode in a fourthembodiment according to the present invention.

FIG. 12A is a cross sectional elevation view illustrative of a novellaser diode of FIG. 11.

FIG. 12B is a diagram illustrative of an energy band gap profile of amultiple quantum well structure of an active layer in a novel laserdiode of FIG. 12A.

FIG. 12C is a diagram illustrative of energy band gap levels of amultiple quantum well structure in a novel laser diode of FIG. 12A.

FIG. 13 a squint view illustrative of a novel laser diode in a fifthembodiment according to the present invention.

FIG. 14A is a cross sectional elevation view illustrative of a novellaser diode of FIG. 13.

FIG. 14B is a diagram illustrative of an energy band gap profile of amultiple quantum well structure of an active layer in a novel laserdiode of FIG. 14A.

FIG. 14C is a diagram illustrative of energy band gap levels of amultiple quantum well structure in a novel laser diode of FIG. 14A.

FIG. 15 a squint view illustrative of a novel laser diode in a sixthembodiment according to the present invention.

FIG. 16A is a cross sectional elevation view illustrative of a novellaser diode of FIG. 15.

FIG. 16B is a diagram illustrative of an energy band gap profile of amultiple quantum well structure of an active layer in a novel laserdiode of FIG. 16A.

FIG. 16C is a diagram illustrative of energy band gap levels of amultiple quantum well structure in a novel laser diode of FIG. 16A.

FIG. 17 a squint view illustrative of a novel laser diode in a seventhembodiment according to the present invention.

FIG. 18A is a cross sectional elevation view illustrative of a novellaser diode of FIG. 15.

FIG. 18B is a diagram illustrative of an energy band gap profile of amultiple quantum well structure of an active layer in a novel laserdiode of FIG. 18A.

FIG. 18C is a diagram illustrative of energy band gap levels of amultiple quantum well structure in a novel laser diode of FIG. 18A.

DISCLOSURE OF THE INVENTION

The present invention provides an active layer structure provided in alight emission device for emitting a light with a predeterminedwavelength. The active layer structure comprises an electroluminescencesection and a carrier accumulation section in alignment with theelectro-luminescence section. The electroluminescence section comprisesa first multiple quantum well structure comprising alternatinglaminations of first well layers showing electroluminescence and firstpotential barrier layers. The first well layers have a first set of auniform energy band gap which corresponds to the predeterminedwavelength. The carrier accumulation section comprises a second multiplequantum well structure comprising alternating laminations of second welllayers and second potential barrier layers, provided that energy bandgap is defined as a difference between a ground level of electrons inconduction band and a ground level of holes in valence band. The secondwell layers have a second set of energy band gaps in a range which isabove the first set of energy band gaps and below a set of forbiddenband widths of the first potential barrier layers. The range of thesecond set of energy band gaps is defined so that the second well layersexhibit carrier accumulations and no electro-luminescence to therebyensure that carriers accumulated in the second well layers are injectedinto the first well layers when the first well layers are deficient incarriers for the electro-luminescence.

The above well layer, in which carriers are accumulated for injectioninto the first well layers when the first well layers are deficient incarriers, makes the first well layers free from being deficient incarriers for the electro-luminescence. This causes almost no variationin refractive index of a laser medium or a considerable reduction invariation in refractive index of a laser medium, This suppresses anactive wavelength chirping.

In the above case, it may be available that the first and second welllayers and the potential barrier layers vary in forbidden band width andin wavelength composition as well as in thickness, so that every thefirst well layers have a uniform energy band gap which corresponds tothe predetermined wavelength, and further that the second set of energyband gaps is in the range which is above the first set of energy bandgaps and below the set of forbidden band widths of the potential barrierlayers, where the range of the second set of energy band gaps is definedso that the second well layers exhibit carrier accumulations and noelectro-luminescence to thereby ensure that carriers accumulated in thesecond well layer is injected into the first well layers when the firstwell layers are deficient in carriers for the electro-luminescence.

Alternatively, it may also be available that the first well layers havea uniform forbidden band width and a uniform wavelength composition aswell as a uniform thickness. Further, the first potential barer layersalso have a uniform forbidden band width and a uniform wavelengthcomposition as well as a uniform thickness. Furthermore, the second welllayers have a uniform forbidden band width and a uniform wavelengthcomposition as well as a uniform thickness. Moreover, the secondpotential barrier layers also have a uniform forbidden band width and auniform wavelength composition as well as a uniform thickness.

In the just above alternative case, it may for example be available thatthe first well layers have a wavelength composition of 1.72 micrometersand a thickness of 5.5 nanometers as well as a first energy band gap of0.80 eV. Further, it may for example be available that the firstpotential barrier layers have a wavelength composition of 1.15micrometers and of a thickness 4 micrometers. Furthermore, the secondwell layers have a wavelength composition of 1.40 micrometers and athickness of 4.0 nanometers as well as a second energy band gap of 0.82eV. Moreover, the second potential barrier layers have a wavelengthcomposition of 1.15 micrometers and of a thickness 6 micrometers.

In the just above case, it may more concretely be available that thefirst well layers are made of +0.8%-strained InGaAs layers. Further, thefirst potential barrier layers are made of non-strained InGaAs layers.Furthermore, the second well layers are made of +0.6%-strained InGaAslayers. Moreover, the second potential barrier layers are made of-0.2%-strained InGaAs layers.

In the above alternative case, it may for example be available that thefirst energy band gap is gradually changed into the second energy bandgap.

The present invention also provides a semiconductor laser deviceincluding an active layer structure which is provided on an opticalguide layer provided on a surface, having a grating structure, of asemiconductor substrate. The active layer structure comprising anelectroluminescence section and a carrier accumulation section inalignment with the electroluminescence section. The electroluminescencesection comprises a first multiple quantum well structure comprisingalternating laminations of first well layers showing electroluminescenceand first potential barrier layers. The first well layers have a firstset of a uniform energy band gap which corresponds to the predeterminedwavelength. The carrier accumulation section comprises a second multiplequantum well structure comprising alternating laminations of second welllayers and second potential barrier layers, provided that energy bandgap is defined as a difference between a ground level of electrons inconduction band and a ground level of holes in valence band. The secondwell layers have a second set of energy band gaps in a range which isabove the first set of energy band gaps and below a set of forbiddenband widths of the first potential barrier layers. The range of thesecond set of energy band gaps is defined so that the second well layersexhibit carrier accumulations and no electro-luminescence to therebyensure that carriers accumulated in the second well layers are injectedinto the first well layers when the first well layers are deficient incarriers for the electroluminescence.

The above well layer, in which carriers are accumulated for injectioninto the first well layers when the first well layers are deficient incarriers, makes the first well layers free from being deficient incarriers for the electro-luminescence. This causes almost no variationin refractive index of a laser medium or a considerable reduction invariation in refractive index of a laser medium. This suppresses anactive wavelength chirping.

In the above case, it may be available that the first and second welllayers and the potential barrier layers vary in forbidden band width andin wavelength composition as well as in thickness, so that every thefirst well layers have a uniform energy band gap which corresponds tothe predetermined wavelength, and further so that the second set ofenergy band gaps is in the range which is above the first set of energyband gaps and below the set of forbidden band widths of the potentialbarrier layers, where the range of the second set of energy band gaps isdefined so that the second well layers exhibit carrier accumulations andno electro-luminescence, to thereby ensure that carriers accumulated inthe second well layer is injected into the first well layers when thefirst well layers are deficient in carriers for theelectro-luminescence.

Alternatively, it may be available that the first well layers have auniform forbidden band width and a uniform wavelength composition aswell as a uniform thickness. Further, the first potential barrier layersalso have a uniform forbidden band width and a uniform wavelengthcomposition as well as a uniform thickness. Furthermore, the second welllayers have a uniform forbidden band width and a uniform wavelengthcomposition as well as a uniform thickness. Moreover, the secondpotential barrier layers also have a uniform forbidden band width and auniform wavelength composition as well as a uniform thickness.

In the just above alternative case, it may be available that the firstwell layers have a wavelength composition of 1.72 micrometers and athickness of 5.5 nanometers as well as a first energy band gap of 0.80eV. Further, the first potential barrier layers have a wavelengthcomposition of 1.15 micrometers and of a thickness 4 micrometers.Furthermore, the second well layers have a wavelength composition of1.40 micrometers and a thickness of 4.0 nanometers as well as a secondenergy band gap of 0.82 eV. Moreover, the second potential barrierlayers have a wavelength composition of 1.15 micrometers and of athickness 6 micrometers.

In the just above case, it may more concretely be available that thefirst well layers are made of +0.8%-strained InGaAs layers. Further, thefirst potential barrier layers are made of non-strained InGaAs layers.Furthermore, the second well layers are made of +0.6%-strained InGaAslayers. Moreover, the second potential barrier layers are made of-0.2%-strained InGaAs layers.

Additionally, it may preferably be available that the first energy bandgap is gradually changed into the second energy band gap.

The present invention provides another active layer structure providedin a light emission device for emitting a light with a predeterminedwavelength. The active layer structure comprises a multiple quantum wellstructure and at least a second well layer. The multiple quantum wellstructure comprises alternating laminations of first well layers showingelectroluminescence and potential barrier layers. The first well layershave a first set of energy band gaps which are uniform and correspondsto the predetermined wavelength, provided that energy band gap isdefined as a difference between a ground level of electrons inconduction band and a ground level of holes in valence band. The secondwell layer is provided within any of the potential barrier layers sothat the second well layer is separated via the potential barer layersfrom the first well layers. The second well layer has a second energyband gap in a range which is above the first set of energy band gaps andbelow a set of forbidden band widths of the potential barrier layers.The range of the second energy band gaps is defined so that the secondwell layer exhibits carrier accumulations and no electro-luminescence tothereby ensure that carriers accumulated in the second well layer areinjected into the first well layers when the first well layers aredeficient in carriers for the electro-luminescence.

The above well layer, in which carriers are accumulated for injectioninto the first well layers when the first well layers are deficient incarriers, makes the first well layers free from being deficient incarriers for the electro-luminescence. This causes almost no variationin refractive index of a laser medium or a considerable reduction invariation in refractive index of a laser medium. This suppresses anactive wavelength chirping.

In the above case, it may be available that the second well layers areprovided within every the potential barrier layers. Further, the secondwell layers have a second set of energy band gaps in the range which isabove the first set of energy band gaps and below a set of forbiddenband widths of the potential barrier layers. Furthermore, the range ofthe second set of energy band gaps is defined so that the second welllayer exhibits carrier accumulations and no electro-luminescence tothereby ensure that carriers accumulated in the second well layer areinjected into the first well layers when the first well layers aredeficient in carriers for the electroluminescence.

Alternatively, it may optionally be available to further provide atleast a third well layer on any interface of the first well layers andthe potential barrier layers so that the third well layer is sandwichedbetween the first well layer and the potential barrier layer. The thirdwell layer has a third energy band gap in the range which is above thefirst set of energy band gaps and below the set of forbidden band widthsof the potential barrier layers. The range of the third energy band gapis defined so that the third well layer exhibits carrier accumulationsand no electro-luminescence to thereby ensure that carriers accumulatedin the third well layer are injected into the first well layers when thefirst well layers are deficient in carriers for the electroluminescence.

In the just above alternative case, it may preferably available that thethird well layers are provided to be sandwiched by every combinations ofthe first well layers and the potential barrier layers so that every thefirst well layers are sandwiched between the third well layers and thepotential barrier layers. Further, the third well layers have a thirdset of energy band gaps in the range which is above the first set ofenergy band gaps and below the set of forbidden band widths of thepotential barrier layers. Furthermore, the range of the third set ofenergy band gaps is defined so that the third well layer exhibitscarrier accumulations and no electro-luminescence to thereby ensure thatcarriers accumulated in the third well layer are injected into the firstwell layers when the first well layers are deficient in carriers for theelectro-luminescence.

Further alternatively, it may optionally be available to further provideat least a third well layer being provided on any interface of the firstwell layers and the potential barrier layers so that the third welllayer is sandwiched between the first well layer and the potentialbarrier layer. The third well layer has a third energy band gap in therange which is above the first set of energy band gaps and below the setof forbidden band widths of the potential barrier layers. The range ofthe third energy band gap is defined so that the third well layerexhibits carrier accumulations and no electroluminescence to therebyensure that carriers accumulated in the third well layer are injectedinto the first well layers when the first well layers are deficient incarriers for the electro-luminescence.

In the just above further alternative case, it may preferably availablethat the third well layers are provided to be sandwiched by everycombinations of the first well layers and the potential barrier layersso that every the first well layers are sandwiched between the thirdwell layers and the potential barrier layers. Further, the third welllayers have a third set of energy band gaps in the range which is abovethe first set of energy band gaps and below the set of forbidden bandwidths of the potential barrier layers. Furthermore, the range of thethird set of energy band gaps is defined so that the third well layerexhibits carrier accumulations and no electroluminescence to therebyensure that carriers accumulated in the third well layer are injectedinto the first well layers when the first well layers are deficient incarriers for the electro-luminescence.

In the above alternative case, it may preferably available that thethird well layers are provided within every the interfaces of the firstwell layers and the potential barrier layers so that every the firstwell layers are sandwiched by the third well layers. The third welllayers have a third set of energy band gaps in the range which is abovethe first set of energy band gaps and below the set of forbidden bandwidths of the potential barrier layers. The range of the second set ofenergy band gaps is defined so that the third well layer exhibitscarrier accumulations and no electro-luminescence to thereby ensure thatcarriers accumulated in the third well layer are injected into the firstwell layers when the first well layers are deficient in carriers for theelectro-luminescence.

In the above further alternative case, it may preferably available thatthe third well layers are provided within every the interfaces of thefirst well layers and the potential barrier layers so that every thefirst well layers are sandwiched by the third well layers. The thirdwell layers have a third set of energy band gaps in the range which isabove the first set of energy band gaps and below the set of forbiddenband widths of the potential barrier layers. The range of the second setof energy band gaps is defined so that the third well layer exhibitscarrier accumulations and no electro-luminescence to thereby ensure thatcarriers accumulated in the third well layer are injected into the firstwell layers when the first well layers are deficient in carriers for theelectro-luminescence.

Still further alternatively, it may optionally be available to furtherprovide at least a fourth well layer at any side of the multiple quantumwell structure so that the fourth well layer is separated via thepotential barrier layers from the first well layers. The fourth welllayer has a fourth energy band gap in the range which is above the firstset of energy band gaps and below the set of forbidden band widths ofthe potential barrier layers. The range of the second energy band gap isdefined so that the fourth well layer exhibits carrier accumulations andno electro-luminescence to thereby ensure that carriers accumulated inthe fourth well layer are injected into the first well layers when thefirst well layers are deficient in carriers for the electroluminescence.

In the just above still further alternative case, it may preferablyavailable that the fourth well layers are provided at opposite sides ofthe multiple quantum well structure so that the multiple quantum wellstructure is positioned between the fourth well layers.

Still more alternatively, it may optionally be available to furtherprovide at least a fourth well layer being provided at any side of themultiple quantum well structure so that the fourth well layer isseparated via the potential barrier layers from the first well layers.The fourth well layer has a fourth energy band gap in the range which isabove the first set of energy band gaps and below the set of forbiddenband widths of the potential barrier layers. The range of the secondenergy band gap is defined so that the fourth well layer exhibitscarrier accumulations and no electro-luminescence to thereby ensure thatcarriers accumulated in the fourth well layer are injected into thefirst well layers when the first well layers are deficient in carriersfor the electro-luminescence.

In the just above still more alternative case, it may preferablyavailable that the fourth well layers are provided at opposite sides ofthe multiple quantum well structure so that the multiple quantum wellstructure is positioned between the fourth well layers.

Yet further alternatively, it may be available that the first and secondwell layers and the potential barrier layers vary in forbidden bandwidth and in wavelength composition as well as in thickness, so thatevery the first well layers have a uniform energy band gap whichcorresponds to the predetermined wavelength, and further so that thesecond energy band gap is in the range which is above the first set ofenergy band gaps and below the set of forbidden band widths of thepotential barrier layers. The range of the second energy band gap isdefined so that the second well layer exhibits carrier accumulations andno electro-luminescence. This ensures that carriers accumulated in thesecond well layer is injected into the first well layers when the firstwell layers are deficient in carriers for the electro-luminescence.

In the above alternative case, it may also be available that the firstand second well layers and the potential barrier layers vary inforbidden band width and in wavelength composition as well as inthickness, so that every the first well layers have a uniform energyband gap which corresponds to the predetermined wavelength, and furtherso that the second set of energy band gaps is in the range which isabove the first set of energy band gaps and below the set of forbiddenband widths of the potential barrier layers. The range of the second setof energy band gaps is defined so that the second well layers exhibitcarrier accumulations and no electro-luminescence. This ensures thatcarriers accumulated in the second well layer is injected into the firstwell layers when the first well layers are deficient in carriers for theelectro-luminescence.

Still moreover alternatively, it may be available that the first welllayers have a uniform forbidden band width and a uniform wavelengthcomposition as well as a uniform thickness. The potential barrier layersalso have a uniform forbidden band width and a uniform wavelengthcomposition as well as a uniform thickness.

Again, in the above alternative case, it may also be available that thefirst well layers have a uniform forbidden band width and a uniformwavelength composition as well as a uniform thickness. The potentialbarrier layers also have a uniform forbidden band width and a uniformwavelength composition as well as a uniform thickness. The second welllayers have a uniform forbidden band width and a uniform wavelengthcomposition as well as a uniform thickness.

Yet moreover alternatively, it may for example be available that thefirst well layers have a wavelength composition of 1.67 micrometers anda thickness of 5.0 nanometers as well as a first energy band gap of 0.80eV. Further, the potential barrier layers have a wavelength compositionof 1.15 micrometers and of a thickness 3 micrometers. Furthermore, thesecond well layer has a wavelength composition of 1.40 micrometers and athickness of 4.0 nanometers as well as a second energy band gap of 1.00eV.

In the just above yet moreover alternative case, the first well layersare made of +0.6%-strained InGaAs layers. The potential barrier layersare made of non-strained InGaAs layers. The second well layer is made ofa non-strained InGaAs layer.

The present invention also provides another semiconductor laser deviceincluding an active layer structure which is provided on an opticalguide layer provided on a surface, having a grating structure, of asemiconductor substrate. The active layer structure comprises a multiplequantum well structure and a multiple quantum well structure. Themultiple quantum well structure comprises alternating laminations offirst well layers showing electroluminescence and potential barrierlayers. The first well layers have a first set of energy band gaps whichare uniform and corresponds to the predetermined wavelength, providedthat energy band gap is defined as a difference between a ground levelof electrons in conduction band and a ground level of holes in valenceband. The multiple quantum well structure is provided within any of thepotential barrier layers so that the second well layer is separated viathe potential barrier layers from the first well layers. The second welllayer has a second energy band gap in a range which is above the firstset of energy band gaps and below a ret of forbidden band widths of thepotential barrier layers. The range of the second energy band gaps isdefined so that the second well layer exhibits carrier accumulations andno electro-luminescence to thereby ensure that carriers accumulated inthe second well layer are injected into the first well layers when thefirst well layers are deficient in carriers for theelectro-luminescence.

The above well layer, in which carriers are accumulated for injectioninto the first well layers when the first well layers are deficient incarriers, makes the first well layers free from being deficient incarriers for the electro-luminescence. This causes almost no variationin refractive index of a laser medium or a considerable reduction invariation in refractive index of a laser medium. This suppresses anactive wavelength chirping.

In the above case, it may preferably be available that the second welllayers are provided within every the potential barrier layers. Thesecond well layers have a second set of energy band gaps in the rangewhich is above the first set of energy band gaps and below a set offorbidden band widths of the potential barrier layers. The range of thesecond set of energy band gaps is defined so that the second well layerexhibits carrier accumulations and no electro-luminescence. This ensuresthat carriers accumulated in the second well layer are injected into thefirst well layers when the first well layers are deficient in carriersfor the electro-luminescence.

Alternatively, it may also be available that the first and second welllayers and the potential barrier layers vary in forbidden band width andin wavelength composition as well as in thickness, so that every thefirst well layers have a uniform energy band gap which corresponds tothe predetermined wavelength, and further so that the second energy bandgap is in the range which is above the first set of energy band gaps andbelow the set of forbidden band widths of the potential barrier layers.The range of the second energy band gap is defined so that the secondwell layer exhibits carrier accumulations and no electro-luminescence.This ensures that carriers accumulated in the second well layer isinjected into the first well layers when the first well layers aredeficient in carriers for the electro-luminescence.

In the above preferable case, it may be available that the first andsecond well layers and the potential barrier layers vary in forbiddenband width and in wavelength composition as well as in thickness, sothat every the first well layers have a uniform energy band gap whichcorresponds to the predetermined wavelength, and further so that thesecond set of energy band gaps is in the range which is above the firstset of energy band gaps and below the set of forbidden band widths ofthe potential barrier layers. The range of the second set of energy bandgaps is defined so that the second well layers exhibit carrieraccumulations and no electro-luminescence. This ensures that carriersaccumulated in the second well layer is injected into the first welllayers when the first well layers are deficient in carriers for theelectro-luminescence.

Further alternatively, it may also be available that the first welllayers have a uniform forbidden band width and a uniform wavelengthcomposition as well as a uniform thickness. The potential barrier layersalso have a uniform forbidden band width and a uniform wavelengthcomposition as well as a uniform thickness.

In the above preferable case, it may more preferably that the first welllayers have a uniform forbidden band width and a uniform wavelengthcomposition as well as a uniform thickness. The potential barrier layersalso have a uniform forbidden band width and a uniform wavelengthcomposition as well as a uniform thickness. The second well layers havea uniform forbidden band width and a uniform wavelength composition aswell as a uniform thickness.

Moreover alternatively, it may for example be available that the firstwell layers have a wavelength composition of 1.67 micrometers and athickness of 5.0 nanometers as well as a first energy band gap of 0.80eV. Further, the potential barrier layers have a wavelength compositionof 1.15 micrometers and of a thickness 3 micrometers. Furthermore, thesecond well layer has a wavelength composition of 1.40 micrometers and athickness of 4.0 nanometers as well as a second energy band gap of 1.00eV.

In the just above moreover alternative case, it may more concretely beavailable that the first well layers are made of +0.6%-strained InGaAslayers. Further, the potential barrier layers are made of non-strainedInGaAs layers. Furthermore, the second well layer is made of anon-strained InGaAs layer.

The present invention provides still another active layer structureprovided in a light emission device for emitting a light with apredetermined wavelength. The active layer structure comprises amultiple quantum well structure and at least a second well layer. Themultiple quantum well structure comprises alternating laminations offirst well layers showing electroluminescence and potential barrierlayers. The first well layers have a first set of energy band gaps whichare uniform and corresponds to the predetermined wavelength, providedthat energy band gap is defined as a difference between a ground levelof electrons in conduction band and a ground level of holes in valenceband. The second well layer is provided on any interface of the firstwell layers and the potential barrier layers so that the second welllayer is sandwiched between the first well layer and the potentialbarrier layer. The second well layer has a second energy band gap in arange which is above the first set of energy band gaps and below a setof forbidden band widths of the potential barrier layers. The range ofthe second energy band gap is defined so that the second well layerexhibits carrier accumulations and no electro-luminescence. This ensuresthat carriers accumulated in the second well layer arc injected into thefirst well layers when the first well layers are deficient in carriersfor the electro-luminescence.

The above well layer, in which carriers are accumulated for injectioninto the first well layers when the first well layers are deficient incarriers, makes the first well layers free from being deficient incarriers for the electro-luminescence. This causes almost no variationin refractive index of a laser medium or a considerable reduction invariation in refractive index of a laser medium. This suppresses anactive wavelength chirping.

It is preferably available that the second well layers are provided tobe sandwiched by every combinations of the first well layers and thepotential barrier layers so that every the first well layers aresandwiched between the second well layers and the potential barrierlayers. The second well layers have a second set of energy band gaps inthe range which is above the first set of energy band gaps and below theset of forbidden band widths of the potential barrier layers. The rangeof the second set of energy band gaps is defined so that the second welllayer exhibits carrier accumulations and no electro-luminescence. Thisensures that carriers accumulated in the second well layer are injectedinto the first well layers when the first well layers are deficient incarriers for the electro-luminescence.

Alternatively, it may be also available that the second well layers areprovided within every the interfaces of the first well layers and thepotential barrier layers so that every the first well layers aresandwiched by the second well layers. The second well layers have asecond set of energy band gaps in the range which is above the first setof energy band gaps and below the set of forbidden band widths of thepotential barrier layers. The range of the second set of energy bandgaps is defined so that the second well layer exhibits carrieraccumulations and no electro-luminescence. This ensures that carriersaccumulated in the second well layer are injected into the first welllayers when the first well layers are deficient in carriers for theelectro-luminescence.

Further alternatively, it may also be available to further provide atleast a third well layer within any of the potential barrier layers sothat the third well layer is separated via the potential barrier layersfrom the first well layers, The third well layer has a third energy bandgap in the range which is above the first set of energy band gaps andbelow the set of forbidden band widths of the potential barrier layers.The range of the third energy band gap is defined so that the third welllayer exhibits carrier accumulations and no electro-luminescence. Thisensures that carriers accumulated in the third well layer are injectedinto the first well layers when the first well layers are deficient incarriers for the electro-luminescence.

In the just above further alternative case, it may preferably beavailable that the third well layers are provided within every thepotential barrier layers. The third well layers have a third set ofenergy band gaps in the range which is above the first set of energyband gaps and below the set of forbidden band widths of the potentialbarrier layers. The range of the third set of energy band gaps isdefined so that the third well layer exhibits carrier accumulations andno electro-luminescence. This ensures that carriers accumulated in thethird well layer are injected into the first well layers when the firstwell layers are deficient in carriers for the electro-luminescence.

Still further alternatively, it may also be available to further provideat least a third well layer being provided within any of the potentialbarrier layers so that the third well layer is separated via thepotential barrier layers from the first well layers. The third welllayer has a third energy band gap in the range which is above the firstset of energy band gaps and below the set of forbidden band widths ofthe potential barrier layers. The range of the third energy band gap isdefined so that the third well layer exhibits carrier accumulations andno electro-luminescence. This ensures that carriers accumulated in thethird well layer are injected into the first well layers when the firstwell layers are deficient in carriers for the electro-luminescence.

In the just above still further alternative case, it may preferably beavailable that the third well layers are provided within every thepotential barrier layers. The third well layers have a third set ofenergy band gaps in the range which is above the first set of energyband gaps and below the set of forbidden band widths of the potentialbarrier layers. The range of the third set of energy band gaps isdefined so that the third well layer exhibits carrier accumulations andno electro-luminescence. This ensures that carriers accumulated in thethird well layer are injected into the first well layers when the firstwell layers are deficient in carriers for the electro-luminescence.

Yet further alternatively, it may also be available to further provideat least a third well layer being provided within any of the potentialbarrier layers so that the third well layer is separated via thepotential barrier layers from the first well layers. The third welllayer has a third energy band gap in the range which is above the firstset of energy band gaps and below the set of forbidden band widths ofthe potential barrier layers. The range of the third energy band gap isdefined so that the third well layer exhibits carrier accumulations andno electro-luminescence. This ensures that carriers accumulated in thethird well layer are injected into the first well layers when the firstwell layers are deficient in carriers for the electro-luminescence.

In the just above yet further alternative case, it may preferably beavailable that the third well layers are provided within every thepotential barrier layers. The third well layers have a third set ofenergy band gaps in the range which is above the first set of energyband gaps and below the set of forbidden band widths of the potentialbarrier layers. The range of the third set of energy band gaps isdefined so that the third well layer exhibits carrier accumulations andno electro-luminescence. This ensures that carriers accumulated in thethird well layer are injected into the first well layers when the firstwell layers are deficient in carriers for the electro-luminescence.

Still more alternatively, it may also be available to further provide atleast a fourth well layer being provided at any side of the multiplequantum well structure so that the fourth well layer is separated viathe potential barrier layers from the first well layers. The fourth welllayer has a fourth energy band gap in the range which is above the firstset of energy band gaps and below the set of forbidden band widths ofthe potential barrier layers. The range of the second energy band gap isdefined so that the fourth well layer exhibits carrier accumulations andno electro-luminescence. This ensures that carriers accumulated in thefourth well layer are injected into the first well layers when the firstwell layers are deficient in carriers for the electro-luminescence.

In the just above still more alternative case, it may preferably beavailable that the fourth well layers are provided at opposite sides ofthe multiple quantum well structure so that the multiple quantum wellstructure is positioned between the fourth well layers.

Further more alternatively, it may also be available to further provideat least a fourth well layer being provided at any side of the multiplequantum well structure so that the fourth well layer is separated viathe potential barrier layers from the first well layers. The fourth welllayer has a fourth energy band gap in the range which is above the firstset of energy band gaps and below the set of forbidden band widths ofthe potential barrier layers. The range of the second energy band gap isdefined so that the fourth well layer exhibits carrier accumulations andno electro-luminescence. This ensures that carriers accumulated in thefourth well layer are injected into the first well layers when the firstwell layers are deficient in carriers for the electro-luminescence.

In the just above further more alternative case, it may preferably beavailable that the fourth well layers are provided at opposite sides ofthe multiple quantum well structure so that the multiple quantum wellstructure is positioned between the fourth well layers.

Moreover, alternatively, it may also be available that the first andsecond well layers and the potential barrier layers vary in forbiddenband width and in wavelength composition as well as in thickness, sothat every the first well layers have a uniform energy band gap whichcorresponds to the predetermined wavelength, and further so that thesecond energy band gap is in the range which is above the first set ofenergy band gaps and below the set of forbidden band widths of thepotential barrier layers. The range of the second energy band gap isdefined so that the second well layer exhibits carrier accumulations andno electro-luminescence. This ensures that carriers accumulated in thesecond well layer is injected into the first well layers when the firstwell layers are deficient in carriers for the electro-luminescence.

In the above preferable case, it may also be available that the firstand second well layers and the potential barrier layers vary inforbidden band width and in wavelength composition as well as inthickness, so that every the first well layers have a uniform energyband gap which corresponds to the predetermined wavelength, and furtherso that the second set of energy band gaps is in the range which isabove the first set of energy band gaps and below the set of forbiddenband widths of the potential barrier layers. The range of the second setof energy band gaps is dewed so that the second well layers exhibitcarrier accumulations and no electro-luminescence. ensures that carriersaccumulated in the second well layer is injected into the first welllayers when the first well layers are deficient in carriers for theelectro-luminescence.

Still moreover, alternatively, it may also be available that the firstwell layers have a uniform forbidden band width and a uniform wavelengthcomposition as well as a uniform thickness. The potential barrier layersalso have a uniform forbidden band width and a uniform wavelengthcomposition as well as a uniform thickness.

In the above preferable case, it may also be available that the firstwell layers have a uniform forbidden band width and a uniform wavelengthcomposition as well as a uniform thickness. The potential barrier layersalso have a uniform forbidden band width and a uniform wavelengthcomposition as well as a uniform thickness. The second well layers havea uniform forbidden band width and a uniform wavelength composition aswell as a uniform thickness.

Preferably, it may for example be available that the first well layershave a wavelength composition of 1.67 micrometers and a thickness of 4.5nanometers as well as a first energy band gap of 0.80 eV. Further, thepotential barrier layers have a wavelength composition of 1.15micrometers and of a thickness 4 micrometers. Furthermore, the secondwell layer has a wavelength composition of 1.30 micrometers and athickness of 6.0 nanometers as well as a second energy band gap of 1.00eV.

In the just above preferable case, it may more concretely be availablethat the first well layers are made of +0.6%-strained InGak layers.Further, the potential barrier layers are made of non-strained InGaAslayers. Furthermore, the second well layer is made of a non-strainedInGaAs layer.

The present invention also provides still another semiconductor laserdevice including an active layer structure which is provided on anoptical guide layer provided on a surface, having a grating structure,of a semiconductor substrate. The active layer structure comprises amultiple quantum well structure and at least a second well layer. Themultiple quantum well structure comprises alternating laminations offirst well layers showing electroluminescence and potential barrierlayers. The first well layers have a first set of energy band gaps whichare uniform and corresponds to the predetermined wavelength, providedthat energy band gap is defined as a difference between a ground levelof electrons in conduction band and a ground level of holes in valenceband. The second well layer is provided on any interface of the firstwell layers and the potential barrier layers so that the second welllayer is sandwiched between the first well layer and the potentialbarrier layer. The second well layer has a second energy band gap in arange which is above the first set of energy band gaps and below a setof forbidden band widths of the potential barrier layers. The range ofthe second energy band gap is defined so that the second well layerexhibits carrier accumulations and no electro-luminescence. This ensuresthat carriers accumulated in the second well layer are injected into thefirst well layers when the first well layers are deficient in carriersfor the electro-luminescence.

The above well layer, in which carriers are accumulated for injectioninto the first well layers when the first well layers are deficient incarriers, makes the first well layers free from being deficient incarriers for the electro-luminescence. This causes almost no variationin refractive index of a laser medium or a considerable reduction invariation in refractive index of a laser medium. This suppresses anactive wavelength chirping.

In the just above case, it may preferably be available that the secondwell layers are provided to be sandwiched by every combinations of thefirst well layers and the potential barrier layers so that every thefirst well layers are sandwiched between the second well layers and thepotential barrier layers. The second well layers have a second set ofenergy band gaps in the range which is above the first set of energyband gaps and below the set of forbidden band widths of the potentialbarrier layers. The range of the second set of energy band gaps isdefined so that the second well layer exhibits carrier accumulations andno electro-luminescence. This ensures that carriers accumulated in thesecond well layer are injected into the first well layers when the firstwell layers are deficient in carriers for the electroluminescence.

Alternatively, it may also be available that the second well layers areprovided within every the interfaces of the first well layers and thepotential barrier layers so that every the first well layers aresandwiched by the second well layers. The second well layers have asecond set of energy band gaps in the range which is above the first setof energy band gaps and below the set of forbidden band widths of thepotential barrier layers. The range of the second set of energy bandgaps is defined so that the second well layer exhibits carrieraccumulations and no electro-luminescence. This ensures that carriersaccumulated in the second well layer are injected into the first welllayers when the first well layers are deficient in carriers for theelectro-luminescence.

Yet alternatively, it may be available that the first and second welllayers and the potential barrier layers vary in forbidden band width andin wavelength composition as well as in thickness, so that every thefirst well layers have a uniform energy band gap which corresponds tothe predetermined wavelength, and further so that the second energy bandgap is in the range which is above the first set of energy band gaps andbelow the set of forbidden band widths of the potential barrier layers.The range of the second energy band gap is defined so that the secondwell layer exhibits carrier accumulations and no electro-luminescence.This ensures that carriers accumulated in the second well layer isinjected into the first well layers when the first well layers aredeficient in carriers for the electro-luminescence.

Still alternatively, it may be available that the first and second welllayers and the potential barrier layers vary in forbidden band width andin wavelength composition as well as in thickness, so that every thefirst well layers have a uniform energy band gap which corresponds tothe predetermined wavelength, and Her so that the second set of energyband gaps is in the range which is above the first set of energy bandgaps and below the set of forbidden band widths of the potential barrierlayers. The range of the second set of energy band gaps is defined sothat the second well layers exhibit carrier accumulations and noelectro-luminescence. This ensures that carriers accumulated in thesecond well layer is injected into the first well layers when the firstwell layers are deficient in carriers for the electro-luminescence.

Still more alternatively, it may be available that the first well layershave a uniform forbidden band width and a uniform wavelength compositionas well as a uniform thickness. The potential barrier layers also have auniform forbidden band width and a uniform wavelength composition aswell as a uniform thickness.

In the above preferable case, it may further be available that the firstwell layers have a uniform forbidden band width and a uniform wavelengthcomposition as well as a uniform thickness. The potential barrier layersalso have a uniform forbidden band width and a uniform wavelengthcomposition as well as a uniform thickness. The second well layers havea uniform forbidden band width and a uniform wavelength composition aswell as a uniform thickness.

Moreover alternatively, it may for example be available that the firstwell layers have a wavelength composition of 1.67 micrometers and athickness of 4.5 nanometers as well as a first energy band gap of 0.8.0eV. The potential barrier layers have a wavelength composition of 1.15micrometers and of a thickness 4 micrometers. The second well layer hasa wavelength composition of 1.30 micrometers and a thickness of 6.0nanometers as well as a second energy band gap of 1.00 eV.

In the just above moreover alternative case, it may more concretely beavailable that the first well layers are made of +0.6%-strained InGaAslayers. Further, the potential barrier layers are made of non-strainedInGaAs layers. Furthermore, the second well layer is made of anon-strained InGaAs layer.

The present invention also provides yet another active layer structureprovided in a light emission device for emitting a light with apredetermined wavelength. The active layer structure comprises amultiple quantum well structure and at least a second well layer. Themultiple quantum well structure comprises alternating laminations offirst well layers showing electroluminescence and potential barrierlayers. The first well layers have a first set of energy band gaps whichare uniform and corresponds to the predetermined wavelength, providedthat energy band gap is defined as a difference between a ground levelof electrons in conduction band and a ground level of holes in valenceband. The second well layer is provided at any side of the multiplequantum well structure so that the second well layer is separated viathe potential barrier layers from the first well layers. The second welllayer has a second energy band gap in a range which is above the firstset of energy band gaps and below a set of forbidden band widths of thepotential barrier layers. The range of the second energy band gap isdefined so that the second well layer exhibits carrier accumulations andno electro-luminescence. This ensures that carriers accumulated in thesecond well layer are injected into the first well layers when the firstwell layers are deficient in carriers for the electro-luminescence.

The above well layer, in which carriers are accumulated for injectioninto the first well layers when the first well layers are deficient incarriers, makes the first well layers free from being deficient incarriers for the electro-luminescence. This causes almost no variationin refractive index of a laser medium or a considerable reduction invariation in refractive index of a laser medium. This suppresses anactive wavelength chirping.

In the above case, it may preferably be available that the second welllayers are provided at opposite sides of the multiple quantum wellstructure so that the multiple quantum well structure is positionedbetween the second well layers.

Alternatively, it may optionally be available to further provide atleast a third well layer within any of the potential barrier layers sothat the third well layer is separated via the potential barrier layersfrom the first well layers. The third well layer has a second energyband gap in a range which is above the first set of energy band gaps andbelow the set of forbidden band widths of the potential barrier layers.The range of the third energy band gaps is defined so that the thirdwell layer exhibits carrier accumulations and no electro-luminescence.This ensures that carriers accumulated in the third well layer areinjected into the first well layers when the first well layers aredeficient in carriers for the electro-luminescence.

In the just above alternative case, it may be available that the thirdwell layers are provided within every the potential barrier layers. Thethird well layers have a second set of energy band gaps in the rangewhich is above the first set of energy band gaps and below the set offorbidden band widths of the potential barrier layers. The range of thethird set of energy band gaps is defined so that the third well layerexhibits carrier accumulations and no electro-luminescence. This ensuresthat carriers accumulated in the third well layer are injected into thefirst well layers when the first well layers are deficient in carriersfor the electro-luminescence.

Further alternatively, it may optionally be available to further provideat least a third well layer being provided within any of the potentialbarrier layers so that the third well layer is separated via thepotential barrier layers from the first well layers. The third welllayer has a second energy band gap in a range which is above the firstset of energy band gaps and below the set of forbidden band widths ofthe potential barrier layers. The range of the third energy band gaps isdefined so that the third well layer exhibits carrier accumulations andno electro-luminescence. This ensures that carriers accumulated in thethird well layer are injected into the first well layers when the firstwell layers are deficient in carriers for the electro-luminescence.

In the just above further alternative case, it may preferably beavailable that the third well layers are provided within every thepotential banner layers. The third well layers have a second set ofenergy band gaps in the range which is above the first set of energyband gaps and below the set of forbidden band widths of the potentialbarrier layers. The range of the third set of energy band gaps isdefined so that the third well layer exhibits carrier accumulations andno electro-luminescence. The ensures that carriers accumulated in thethird well layer are injected into the first well layers when the firstwell layers are deficient in carriers for the electro-luminescence.

Furthermore alternatively, it may optionally be available to furtherprovide at least a fourth well layer being provided on any interface ofthe first well layers and the potential barrier layers so that thefourth well layer is sandwiched between the first well layer and thepotential barrier layer. The fourth well layer has a fourth energy bandgap in the range which is above the first set of energy band gaps andbelow the set of forbidden band widths of the potential barrier layers.The range of the fourth energy band gap is defined so that the fourthwell layer exhibits carrier accumulations and no electro-luminescence.This ensures that carriers accumulated in the fourth well layer areinjected into the first well layers when the first well layers aredeficient in carriers for the electro-luminescence.

In the above furthermore alternative case, it may optionally beavailable that the fourth well layers are provided to be sandwiched byevery combinations of the first well layers and the potential barrierlayers so that every the first well layers are sandwiched between thefourth well layers and the potential barrier layers. The fourth welllayers have a fourth set of energy band gaps in the range which is abovethe first set of energy band gaps and below the set of forbidden bandwidths of the potential barrier layers. The range of the fourth set ofenergy band gaps is defined so that the fourth well layer exhibitscarrier accumulations and no electro-luminescence. This ensures thatcarriers accumulated in the fourth well layer are injected into thefirst well layers when the first well layers are deficient in carriersfor the electro-luminescence.

In the above furthermore alternative case, it may also optionally beavailable that the fourth well layers are provided within every theinterfaces of the first well layers and the potential barrier layers sothat every the first well layers are sandwiched by the fourth welllayers. The fourth well layers have a fourth set of energy band gaps inthe range which is above the first set of energy band gaps and below theset of forbidden band widths of the potential barrier layers. The rangeof the fourth set of energy band gaps is defined so that the fourth welllayer exhibits carrier accumulations and no electro-luminescence. Thisensures that carriers accumulated in the fourth well layer are injectedinto the first well layers when the first well layers are deficient incarriers for the electro-luminescence.

In the above preferable case, it may be available to further provide atleast a fourth well layer on any interface of the first well layers andthe potential barrier layers so that the fourth well layer is sandwichedbetween the first well layer and the potential barrier layer. The fourthwell layer has a fourth energy band gap in the range which is above thefirst set of energy band gaps and below the set of forbidden band widthsof the potential barrier layers. The range of the fourth energy band gapis defined so that the fourth well layer exhibits carrier accumulationsand no electro-luminescence. This ensures that carriers accumulated inthe fourth well layer are injected into the first well layers when thefirst well layers are deficient in carriers for theelectro-luminescence.

In the just above case, it may preferably available that the fourth welllayers are provided to be sandwiched by every combinations of the firstwell layers and the potential barrier layers so that every the firstwell layers are sandwiched between the fourth well layers and thepotential barrier layers. The fourth well layers have a fourth set ofenergy band gaps in the range which is above the first set of energyband gaps and below the set of forbidden band widths of the potentialbarrier layers. The range of the fourth set of energy band gaps isdefined so that the fourth well layer exhibits carrier accumulations andno electro-luminescence. This ensures that carriers accumulated in thefourth well layer are injected into the first well layers when the firstwell layers are deficient in carriers for the electro-luminescence.

In the above preferable case, it may also be available that the fourthwell layers are provided within every the interfaces of the first welllayers and the potential barrier layers so that every the first welllayers are sandwiched by the fourth well layers. The fourth well layershave a fourth set of energy band gaps in the range which is above thefirst set of energy band gaps and below the set of forbidden band widthsof the potential barrier layers. The range of the fourth set of energyband gaps is defined so that the fourth well layer exhibits carrieraccumulations and no electro-luminescence. This ensures that carriersaccumulated in the fourth well layer are injected into the first welllayers when the first well layers are deficient in carriers for theelectro-luminescence.

Yet further, alternatively, it may be available that the first andsecond well layers and the potential barrier layers vary in forbiddenband width and in wavelength composition as well as in thickness, sothat every the first well layers have a uniform energy band gap whichcorresponds to the predetermined wavelength, and further so that thesecond energy band gap is in the range which is above the first set ofenergy band gaps and below the set of forbidden band widths of thepotential barrier layers. The range of the second energy band gap isdefined so that the second well layer exhibits carrier accumulations andno electro-luminescence. This ensures that carriers accumulated in thesecond well layer is injected into the first well layers when the firstwell layers are deficient in carriers for the electro-luminescence.

Still more, alternatively, it may be available that the first welllayers have a uniform forbidden band width and a uniform wavelengthcomposition as well as a uniform thickness. The potential barrier layersalso have a forbidden band width and a uniform wavelength composition aswell as a uniform thickness.

In the just above still more alternative case, it may for example beavailable that the first well layers have a wavelength composition of1.67 micrometers and a thickness of 5.0 nanometers as well as a firstenergy band gap of 0.8.0 eV. Further, the potential barrier layers havea wavelength composition of 1.15 micrometers and of a thickness 4micrometers. Furthermore, the second well layer has a wavelengthcomposition of 1.30 micrometers and a thickness of 50 nanometers as wellas a second energy band gap which corresponds to the wavelengthcomposition of 1.30 micrometers.

In the just above case, it may more concretely be available that thefirst well layers are made of +0.6%-strained InGaAs layers. Further, thepotential barrier layers are made of non-strained InGaAs layers.Furthermore, the second well layer is made of a non-strained InGaAslayer.

The present invention also provides yet another semiconductor laserdevice including an active layer structure which is provided on anoptical guide layer provided on a surface, having a grating structure,of a semiconductor substrate. The active layer structure comprises amultiple quantum well structure and at least a second well layer. Themultiple quantum well structure comprises alternating laminations offirst well layers showing electroluminescence and potential barrierlayers. The first well layers have a first set of energy band gaps whichare uniform and corresponds to the predetermined wavelength, providedthat energy band gap is defined as a difference between a ground levelof electrons in conduction band and a ground level of holes in valenceband. The second well layer is provided at any side of the multiplequantum well structure so that the second well layer is separated viathe potential barrier layers from the first well layers. The second welllayer has a second energy band gap in a range which is above the firstset of energy band gaps and below a set of forbidden band widths of thepotential barrier layers. The range of the second energy band gap isdefined so that the second well layer exhibits carrier accumulations andno electro-luminescence. This ensures that carriers accumulated in thesecond well layer are injected into the first well layers when the firstwell layers are deficient in carriers for the electroluminescence.

The above well layer, in which carriers are accumulated for injectioninto the first well layers when the first well layers are deficient incarriers, makes the first well layers free from being deficient incarriers for the electro-luminescence. This causes almost no variationin refractive index of a laser medium or a considerable reduction invariation in refractive index of a laser medium. This suppresses anactive wavelength chirping.

In the above case, it may preferably be available that the second welllayers are provided at opposite sides of the multiple quantum wellstructure so that the multiple quantum well structure is positionedbetween the second well layers.

Alternatively, it may be available that the first and second well layersand the potential barrier layers vary in forbidden band width and inwavelength composition as well as in thickness, so that every the firstwell layers have a uniform energy band gap which corresponds to thepredetermined wavelength, and further so that the second energy band gapis in the range which is above the first set of energy band gaps andbelow the set of forbidden band widths of the potential barrier layers.The range of the second energy band gap is defined so that the secondwell layer exhibits carrier accumulations and no electro-luminescence.This ensures that carriers accumulated in the second well layer isinjected into the first well layers when the first well layers aredeficient in carriers for the electro-luminescence.

Further alternatively, it may be available that the first well layershave a uniform forbidden band width and a uniform wavelength compositionas well as a uniform thickness. The potential barrier layers also have auniform forbidden band width and a uniform wavelength composition aswell as a uniform thickness.

In the just above further alternative case, it may for example beavailable that the first well layers have a wavelength composition of1.67 micrometers and a thickness of 5.0 nanometers as well as a firstenergy band gap of 0.8.0 eV. Further, the potential barrier layers havea wavelength composition of 1.15 micrometers and of a thickness 4micrometers. Furthermore, the second well layer has a wavelengthcomposition of 1.30 micrometers and a thickness of 50 nanometers as wellas a second energy band gap which corresponds to the wavelengthcomposition of 1.30 micrometers.

In the just above case, it may more concretely be available that thefirst well layers are made of +0.6%-strained InGaAs layers. Further, thepotential barrier layers are made of non-strained InGaAs layers.Furthermore, the second well layer is made of a non-strained InGaAslayer.

EMBODIMENTS

A first embodiment according to the present invention will be describedwith reference to FIGS. 4, 5A-5C and 6A-6C. A novel laser diode isprovided as illustrated in FIGS. 4 and 5A The novel laser diode isformed on an n-type InP substrate 1. A surface of the n-type InPsubstrate 1 is formed with a grating 9. An optical guide layer 2 isprovided which extends over the grating 9. The optical guide layer 2 ismade of InGaAs which has a wavelength composition of 1.2 micrometers.The optical guide layer 2 has a thickness of 0.1 micrometers. An activelayer is provided which extends over the optical guide layer 2. Theactive layer has a multiple quantum well structure including eightquantum wells. The active layer comprises an elector-luminescence region3-A and a carrier accumulation region 3-B. An optical guide layer 4 isprovided which extends over the active layer 3-A and 3-B. The opticalguide layer 4 is made of InGaAsP which has a wavelength composition of1.2 micrometers. The optical guide layer 4 has a thickness of 0.1micrometers. A p-type InP cladding layer 5 is provided, which extendsover the optical guide layer 4. The p-type InP cladding layer 5 has athickness of 0.05 micrometers and a carrier concentration of 5×10¹⁷cm⁻³. The above thickness are of the individual layers over theelector-luminescence region 3-A A p-type InP layer 6 is provided whichextends over the p-type InP cladding layer 5. The p-type InP layer 6 hasa thickness of 1.3 micrometers and a carrier concentration of 5×10¹⁷cm⁻³. A p-type InGaAsP contact layer 7 is provided which extends overthe p-type grip layer 6. The p-type InGaAsP contact layer 7 has athickness of 0.25 micrometers and a carrier concentration of 8×10¹⁸cm⁻³. A p-type electrode is provided on the p-type InGaAsP contact layer7. The above carrier concentration of the p-type InGaAsP contact layer 7is sufficiently high to form an ohmic contact.

The following descriptions will focus on the multiple quantum wellstructure of the active layer over the electroluminescence region 3-Aand the carrier accumulation region 3-B which are smoothly coupled toeach other. FIG. 5B is a diagram illustrative of an energy band gapprofile of a multiple quantum well structure in an electroluminescenceregion of an active layer in a novel laser diode of FIG. 5A. Themultiple quantum well structure in the electroluminescence region 3-Acomprises alternating laminations of electroluminescence well layers 31and potential barrier layers 32. The electroluminescence well layer 31has a wavelength composition of 1.72 micrometers and a thickness of 5.5nanometers. The electroluminescence well layer 31 comprises+0.8%-strained InGaAs layer. The potential barrier layer 32 has awavelength composition of 1.15 micrometers and a thickness of 4micrometers. The potential barrier layer 32 comprises non-strainedInGaAs layer. As a result, the electroluminescence well layer 31 has anenergy band gap E_(g1) of 0.80 eV, where energy band gap is defined as adifference between ground levels of electrons in a conduction band andholes in a valence band. The energy band gap E_(g1) of 0.80 eV of theelectroluminescence well layer 31 is converted into a compositionwavelength of 1.55 micrometers.

FIG. 5C is a diagram illustrative of an energy band gap profile of amultiple quantum well structure in the carrier accumulation region ofthe active layer in the novel laser diode of FIG. 5A. The multiplequantum well structure in the carrier accumulation region 3-B comprisesalternating laminations of carrier accumulation well layers 33 andpotential barrier layers 34. The carrier accumulation well layer 33 hasa wavelength composition of 1.66 micrometers and a thickness of 4.8nanometers. The carrier accumulation well layer 33 comprises+0.6%-strained InGaAs layer. The potential barrier layer 34 has awavelength composition of 1.15 micrometers and a thickness of 6micrometers. The potential barrier layer 34 comprises -0.2%-strainedInGaAs layer. As a result, the carrier accumulation well layer 33 has anenergy band gap E_(g2) of 0.82 eV, where energy band gap is defined as adifference between ground levels of electrons in a conduction band andholes in a valence band. The energy band gap E_(g2) of 0.82 eV of thecarrier accumulation well layer 33 is converted into a compositionwavelength of 1.52 micrometers.

FIG. 5D is a diagram illustrative of energy band gap levels of themultiple quantum well structure over the electroluminescence region andthe carrier accumulation region in a novel laser diode of FIG. 5A Theenergy band gap level E_(g1) of 0.80 eV of the electroluminescence welllayer 31 is smoothly shifted into the energy band gap E_(g2) of 0.82 eVof the carrier accumulation well layer 33. The electro-luminescence welllayer 31 having the energy band gap level E_(g1) of 0.80 eV showselectro-luminescence at a wavelength of 1.55 micrometers. On the otherhand, the carrier accumulation well layers 33 exhibit carrieraccumulations without any electro-luminescence to thereby ensure thatcarriers accumulated in said second well layers are injected into saidfirst well layers when said first well layers are deficient in carriersfor said electro-luminescence. The above carrier accumulation welllayer, in which carriers are accumulated for injection into the firstwell layers when the first well layers are deficient in carriers, makesthe first well layers free from being deficient in carriers for theelectro-luminescence. This causes almost no variation in refractiveindex of a laser medium or a considerable reduction in variation inrefractive index of a laser medium. This suppresses an active wavelengthchirping.

The following descriptions are directed to the fabrication processes forthe above described novel laser diode with reference to FIGS. 6A through6C. In the fabrication processes, a metal organic chemical vapor phaseepitaxy method is used with selective growth methods using dielectricmasks varying in its widths in order to grow thin epitaxial layershaving modulations in thickness. The metal organic chemical vaporepitaxy method is disclosed, for example, in O. Kayser "Selective growthof InP/GaInAs in LP-MOVPE and MOMBE/CBE" Journal of Crystal Growth, 107,1991, pp. 989-998. The selective growth methods using dielectric masksvarying in its widths is disclosed, for example, in E. Colas et al.,"Lateral and Longitudinal Patterning of semiconductor structure bycrystal growth on non-planar and dielectric-masked GaAs substrate:application o thickness-modulated wave-guide structures", Journal ofCrystal Growth, 107, 1991, pp. 226-230. An optical modulation multiplequantum well distributed fed back laser diode is disclosed, for example,in Kato et al. "Novel MQW DFB LASER DIODE/MODULATOR INTEGRATED LIGHTSOURCE USING BAND GAP ENERGY CONTROL EPTAXTIAL GROWTH TECHNIQUE",European Conference on Optical Communication, WeB-7-1, 1991, pp.429-432. A wavelength tunable multiple quantum well distributed Braggreflector laser diode using band gap energy control is disclosed, forexample, in S. Takano et al., "1.55 μm WAVELENGTH-TUNABLE MQW-DBR-LDsBANDGAP ENERGY CONTROL IN ALL SELECTIVE MOVPE GROWTH", EuropeanConference on Optical Communication, TuB-5-3, 1992, pp. 177-180.

With reference to FIG. 6A, an n-type InP substrate 1 is prepared, whichhas a surface formed with a λ/4-shifted grating 9. A pair of dielectricmasks M1 is placed over the grating 9. Each of the paired dielectricmasks MI comprises a wide portion on the electro-luminescence region3-A, an intermediate portion on an intermediate region between theelectroluminescence region 3-A and the carrier accumulation region 3-B,and a slender portion on the carrier accumulation region 3-B. Theintermediate portion proportionally varies in width. The wide portion ofthe mask M1 has a width of 12 micrometers and a length of 250micrometers. The slender portion of the mask M1 has a width of 10micrometers and a length of 300 micrometers. The intermediate portion ofthe mask M1 has a length of about 20 micrometers. The above paired masksM1 are arranged to have a constant distance between them. The constantdistance between the paired masks Ml is 1.5 micrometers.

The optical guide layer 2, the active layer 3, the optical guide layer 4and the cladding layer 5 are in turn grown using the above masks Ml. Thethickness of the individually grown layers 2, 3, 4 and 5 are vary sothat the thickness of the layer on the electro-luminescence region 3-Ais larger than the thickness of the layer on the carrier accumulationregion 3-B. The wavelength composition is small when the thicknessthereof is thin, while the wavelength composition is large when thethickness thereof is thick.

The above used masks M1 are removed, in place of which a pair of anothermasks M2 is prepared. The masks M2 differs from the masks M1 in thedistance between them. Of the masks M1, the distance is about 1.5micrometers as described above, whilst of the masks M2, the distance isabout 5 micrometers.

The InP layer 6 and the contact layer 7 are in turn grown using themasks M2. A dielectric layer 8 is formed, which covers the entire topsurface of the substrate 1. A p-electrode 10 is selectively formed onthe dielectric layer 8. An n-electrode 11 is formed entirely on thebottom of the substrate 1. The above laser diode is then fabricated.

It was confirmed that when the above laser diode is subjected to adirect modulation at -20%dB, then the above laser diode shows a reducedactive wavelength chirping Δλ of not more than 0.2 nanometers. Bycontrast, when the conventional laser diode is subjected to a directmodulation at -20%dB, then the conventional laser diode shows a reducedactive wavelength chirping Δλ of not more than 0.4 nanometers. Theactive wavelength chirping, that the above novel laser diode shows, isabout a half of the active wavelength chirping, that the aboveconventional laser diode shows. It was also confirmed that the abovenovel laser diode has a threshold current of 10 mA. The measuredefficiency of the above novel laser diode is not less than 0.2 W/A. Theoutput of the above novel laser diode is not less than 20 mW. The abovegrowth method allows that the optical guide layers have reduced internallosses and show high outputs. The above fabrication processes allows ahigh yield and a reduction in the cost of manufacturing.

A second embodiment according to the present invention will be describedwith reference to FIGS. 7 and 8A-8C. A novel laser diode is provided asillustrated in FIGS. 7 and 8A The novel laser diode is formed on ann-type InP substrate 1. A surface of the n-type InP substrate 1 isformed with a grating 9. An optical guide layer 2 is provided whichextends over the grating 9. The optical guide layer 2 is made of InGaAswhich has a wavelength composition of 1.2 micrometers. The optical guidelayer 2 has a thickness of 0.1 micrometers. An active layer is providedwhich extends over the optical guide layer 2. The active layer has amultiple quantum well structure including eight quantum wells. Anoptical guide layer 4 is provided which extends over the active layer 3.The optical guide layer 4 is made of InGaAsP which has a wavelengthcomposition of 1.2 micrometers. The optical guide layer 4 has athickness of 0.1 micrometers. A p-type InP cladding layer 5 is provided,which extends over the optical guide layer 4. The p-type InP claddinglayer 5 has a thickness of 0.05 micrometers and a carrier concentrationof 5×10¹⁷ cm⁻³. A p-type InP layer 6 is provided which extends over thep-type InP cladding layer 5. The p-type InP layer 6 has a thickness of1.3 micrometers and a carrier concentration of 5×10¹⁷ cm⁻³. A p-typeInGaAsP contact layer 7 is provided which extends over the p-type InPlayer 6. The p-type InGaAsP contact layer 7 has a thickness of 0.25micrometers and a carrier concentration of 8×10¹⁸ cm⁻³. A p-typeelectrode is provided on the p-type InGaAsP contact layer 7. The abovecarrier concentration of the p-type InGaAsP contact layer 7 issufficiently high to form an ohmic contact.

The following descriptions will focus on the multiple quantum wellstructure of the active layer 3 with reference to FIGS. 8B and 8C. Themultiple quantum well structure comprises eight periods of the followinglamination structures. An electro-luminescence well layer 35 is providedin contact with a bulk region serving as a potential barrier. Apotential barrier layer 36 is provided in contact with theelectro-luminescence well layer 35. A carrier accumulation well layer 37is provided in contact with the potential barrier layer 36. Thepotential barrier layer 36 is provided in contact with the carrieraccumulation well layer 37. The electro-luminescence well layer 35 isagain provided in contact with the potential barrier layer 36. Thepotential barrier layer 36 is provided in contact with theelectro-luminescence well layer 35. The carrier accumulation well layer37 is provided in contact with the potential barrier layer 36.

Namely, each of the electro-luminescence well layers 35 is sandwichedbetween the potential barrier layers 36 as well as each of the carrieraccumulation well layers 37 is also sandwiched between the potentialbarrier layers 36 so that each of the electro-luminescence well layers35 is separated via the potential barrier layer 36 from each of thecarrier accumulation well layers 37.

The electro-luminescence well layer 35 comprises a +0.6%-strained InGaAslayer which has a thickness of 5.0 nanometers and a wavelengthcomposition of 1.67 micrometers. The potential barrier layer 36comprises a non-strained InGaAs which has a thickness of 3.0 micrometersand a wavelength composition of 1.15 micrometers. The carrieraccumulation well layer 37 comprises a non-strained InGaAs layer whichhas a thickness of 4.0 nanometers and a wavelength composition of 1.40micrometers. The electroluminescence well layer 35 has an energy bandgap E_(g1) of 0.8.0 eV, where energy band gap is defined as a differencebetween a ground level of electrons in conduction band and a groundlevel of holes in valence band. The energy band gap E_(g1) of 0.8.0 eVof the electro-luminescence well layer 35 is converted into a wavelengthof 1.55 micrometers. The carrier accumulation well layer 37 has anenergy band gap E_(g2) of 1.00 eV, where energy band gap is defined as adifference between a ground level of electrons in conduction band and aground level of holes in valence band. The energy band gap E_(g2) of1.00 eV of the carrier accumulation well layer 37 is converted into awavelength of 1.25 micrometers.

The above electro-luminescence well layer 35 having the small energyband gap E_(g1) of 0.8.0 eV shows electro-luminescence at a wavelengthof 1.55 micrometers. On the other hand, the carrier accumulation welllayers 37 exhibit carrier accumulations without any electro-luminescenceto thereby ensure that carriers accumulated in said second well layersare injected into said first well layers when said first well layers aredeficient in carriers for said electro-luminescence. The above earneraccumulation well layer, in which carriers are accumulated for injectioninto the electro-luminescence well layers when the electro-luminescencewell layers are deficient in carriers, makes the electro-luminescencewell layers free from being deficient in carriers for theelectro-luminescence. This causes almost no variation in refractiveindex of a laser medium or a considerable reduction in variation inrefractive index of a laser medium. This suppresses an active wavelengthchirping.

It was confirmed that when the above laser diode is subjected to adirect modulation at -20%dB, then the above laser diode shows a reducedactive wavelength chirping Δλ of not more than 0.2 nanometers. Bycontrast, when the conventional laser diode is subjected to a directmodulation at -20%dB, then the conventional laser diode shows a reducedactive wavelength chirping Δλ of not more than 0.4 nanometers. Theactive wavelength chirping, that the above novel laser diode shows, isabout a half of the active wavelength chirping, that the aboveconventional laser diode shows. It was also confirmed that the abovenovel laser diode has a threshold current of 10 mA The measuredefficiency of the above novel laser diode is not less than 0.2 W/A. Theoutput of the above novel laser diode is not less than 20 mW. The abovegrowth method allows that the optical guide layers have reduced internallosses and show high outputs. The above fabrication processes allows ahigh yield and a reduction in the cost of manufacturing.

A third embodiment according to the present invention will be describedwith reference to FIGS. 9 and 10A-10C. A novel laser diode is providedas illustrated in FIGS. 9 and 10A The novel laser diode is formed on ann-type InP substrate 1. A surface of the n-type InP substrate 1 isformed with a grating 9. An optical guide layer 2 is provided whichextends over the grating 9. The optical guide layer 2 is made of InGaAswhich has a wavelength composition of 1.2 micrometers. The optical guidelayer 2 has a thickness of 0.1 micrometers. An active layer is providedwhich extends over the optical guide layer 2. The active layer has amultiple quantum well structure including eight quantum wells. Anoptical guide layer 4 is provided which extends over the active layer 3.The optical guide layer 4 is made of InGaAsP which has a wavelengthcomposition of 1.2 micrometers. The optical guide layer 4 has athickness of 0.1 micrometers. A p-type InP cladding layer 5 is provided,which extends over the optical guide layer 4. The p-type InP claddinglayer 5 has a thickness of 0.05 micrometers and a carrier concentrationof 5×10¹⁷ cm⁻³. A p-type InP layer 6 is provided which extends over thep-type In cladding layer 5. The p-type InP layer 6 has a thickness of1.3 micrometers and a carrier concentration of 5×10¹⁷ cm⁻³. A p-typeInGaAsP contact layer 7 is provided which extends over the p-type InPlayer 6. The p-type InGaAsP contact layer 7 has a thickness of 0.25micrometers and a carrier concentration of 8×10¹⁸ cm⁻³. A p-typeelectrode is provided on the p-type InGaAsP contact layer 7. The abovecarrier concentration of the p-type InGaAsP contact layer 7 issufficiently high to form an ohmic contact.

The following descriptions will focus on the multiple quantum wellstructure of the active layer 3 with reference to FIGS. 10B and 10C. Themultiple quantum well structure comprises eight periods of the followinglamination structures. A carrier accumulation layer 38 is provided incontact with a bulk region serving as a potential barrier. Anelectro-luminescence well layer 39 is provided in contact with thecarrier accumulation layer 38. A potential barrier layer 40 is providedin contact with the electro-luminescence well layer 39. The carrieraccumulation well layer 38 is provided in contact with the potentialbarrier layer 40.

Namely, each of the electro-luminescence well layers 39 is sandwichedbetween the potential barrier layer 40 and the carrier accumulationlayer 38. In other word, each of the carrier accumulation well layer 38is sandwiched between the potential barrier layer 40 and theelectro-luminescence well layer 39.

The electro-luminescence well layer 39 comprises a +0.6%-strained InGaAslayer which has a thickness of 4.5 nanometers and a wavelengthcomposition of 1.67 micrometers. The potential barrier layer 40comprises a non-strained InGaAs which has a thickness of 4.0 micrometersand a wavelength composition of 1.15 micrometers. The carrieraccumulation well layer 38 comprises a non-strained InGaAs layer whichhas a thickness of 6.0 nanometers and a wavelength composition of 1.30micro-meters. The electroluminescence well layer 39 has an energy bandgap E_(g1) of 0.8.0 eV, where energy band gap is defined as a differencebetween a ground level of electrons in conduction band and a groundlevel of holes in valence band. The energy band gap E_(g1) of 0.8.0 eVof the electro-luminescence well layer 39 is converted into a wavelengthof 1.55 micrometers. The carrier accumulation well layer 38 has anenergy band gap E_(g2) of 1.00 eV, where energy band gap is defined as adifference between a ground level of electrons in conduction band and aground level of holes in valence band. The energy band gap E_(g2) of1.00 eV of the carrier accumulation well layer 38 is converted into awavelength of 1.25 micrometers.

The above electro-luminescence well layer 39 having the small energyband gap E_(g1) of 0.8.0 eV shows electro-luminescence at a wavelengthof 1.55 micrometers. On the other hand, the carrier accumulation welllayers 38 exhibit carrier accumulations without any electro-luminescenceto thereby ensure that carriers accumulated in said second well layersare injected into said first well layers when said first well layers aredeficient in carriers for said electro-luminescence. The above carrieraccumulation well layer, in which carriers are accumulated for injectioninto the electro-luminescence well layers when the electro-luminescencewell layers are deficient in carriers, makes the electro-luminescencewell layers free from being deficient in carriers for theelectro-luminescence. This causes almost no variation in refractiveindex of a laser medium or a considerable reduction in variation inrefractive index of a laser medium. This suppresses an active wavelengthchirping.

It was confirmed that when the above laser diode is subjected to adirect modulation at -20%dB, then the above laser diode shows a reducedactive wavelength chirping Δλ of not more than 0.2 nanometers. Bycontrast, when the conventional laser diode is subjected to a directmodulation at -20%dB, then the conventional laser diode shows a reducedactive wavelength chirping Δλ of not more than 0.4 nanometers. Theactive wavelength chirping, that the above novel laser diode shows, isabout a half of the active wavelength chirping, that the aboveconventional laser diode shows. It was also confirmed that the abovenovel laser diode has a threshold current of 10 mA. The measuredefficiency of the above novel laser diode is not less than 0.2 W/A. Theoutput of the above novel laser diode is not less than 20 mW. The abovegrowth method allows that the optical guide layers have reduced internallosses and show high outputs. The above fabrication processes allows ahigh yield and a reduction in the cost of manufacturing.

A fourth embodiment according to the present invention will be describedwith reference to FIGS. 11 and 12A-12C. A novel laser diode is providedas illustrated in FIGS. 11 and 12A. The novel laser diode is formed onan n-type InP substrate 1. A surface of the n-type InP substrate 1 isformed with a grating 9. An optical guide layer 2 is provided whichextends over the grating 9. The optical guide layer 2 is made of InGaAswhich has a wavelength composition of 1.2 micrometers. The optical guidelayer 2 has a thickness of 0.1 micrometers. An active layer is providedwhich extends over the optical guide layer 2. The active layer has amultiple quantum well structure including eight quantum wells. Anoptical guide layer 4 is provided which extends over the active layer 3.The optical guide layer 4 is made of InGaAsP which has a wavelengthcomposition of 1.2 micrometers. The optical guide layer 4 has athickness of 0.1 micrometers. A p-type InP cladding layer 5 is provided,which extends over the optical guide layer 4. The p-type InP claddinglayer 5 has a thickness of 0.05 micrometers and a carrier concentrationof 5×10¹⁷ cm⁻³. A p-type InP layer 6 is provided which extends over thep-type InP cladding layer 5. The p-type InP layer 6 has a thickness of1.3 micrometers and a carrier concentration of 5×10¹⁷ cm ³. A p-typeInGaAsP contact layer 7 is provided which extends over the p-type InPlayer 6. The p-type InGaAsP contact layer 7 has a thickness of 0.25micrometers and a carrier concentration of 8×10¹⁸ cm⁻³. A p-typeelectrode is provided on the p-type InGaAsP contact layer 7. The abovecarrier concentration of the p-type InGaAsP contact layer 7 issufficiently high to form an ohmic contact.

The following descriptions will focus on the multiple quantum wellstructure of the active layer 3 with reference to FIGS. 12B and 12C. Themultiple quantum well structure comprises eight periods of alternatinglaminations of electro-luminescence layers 31 and potential barrierlayers 32. A first carrier accumulation layer 41 is provided under theabove alternating laminations of the electro-luminescence layers 31 andthe potential barrier layers 32. The first carrier accumulation layer 41is separated from the potential barrier layer 32 from theelectro-luminescence layers 31. A second carrier accumulation layer 42is provided over the above alternating laminations of theelectro-luminescence layers 31 and the potential barrier layers 32. Thesecond carrier accumulation layer 42 is separated from the potentialbarrier layer 32 from the electro-luminescence layers 31.

Namely, the above alternating laminations of the electro-luminescencelayers 31 and the potential barrier layers 32 is sandwiched between thefirst and second carrier accumulation layers 41 and 42.

The electro-luminescence well layer 31 comprises a +0.6%-strained InGaAslayer which has a thickness of 5.0 nanometers and a wavelengthcomposition of 1.67 micrometers. The potential barrier layer 32comprises a non-strained InGaAs which has a thickness of 4.0 micrometersand a wavelength composition of 1.15 micrometers. The first carrieraccumulation well layer 41 has a double hetero structure, and comprisesa non-strained InGaAs layer which has a thickness of 50 nanometers and awavelength composition of 1.30 micrometers. The second carrieraccumulation well layer 42 has a double hetero structure, and comprisesa non-strained InGaAs layer which has a thickness of 50 nanometers and awavelength composition of 1.30 micrometers. The electro-luminescencewell layer 31 has an energy band gap E_(g1) of 0.8.0 eV, where energyband gap is defined as a difference between a ground level of electronsin conduction band and a ground level of holes in valence band. Theenergy band gap E_(g1) of 0.8.0 eV of the electro-luminescence welllayer 31 is converted into a wavelength of 1.55 micrometers. The firstand second carrier accumulation well layers 41 and 42 have an energyband gap which is converted into a wavelength of 1.30 micrometers.

The above electro-luminescence well layer 31 having the small energyband gap E_(g1) of 0.8.0 eV shows electro-luminescence at a wavelengthof 1.55 micrometers. On the other hand, the first and second carrieraccumulation well layers 41 and 42 exhibit carrier accumulations withoutany electro-luminescence to thereby ensure that carriers accumulated insaid second well layers are injected into said first well layers whensaid first well layers are deficient in carriers for saidelectro-luminescence. The above carrier accumulation well layers, inwhich carriers are accumulated for injection into theelectro-luminescence well layers when the electro-luminescence welllayers are deficient in carriers, makes the electro-luminescence welllayers free from being deficient in carriers for theelectro-luminescence. This causes almost no variation in refractiveindex of a laser medium or a considerable reduction in variation inrefractive index of a laser medium. This suppresses an active wavelengthchirping.

It was confirmed that when the above laser diode is subjected to adirect modulation at -20%dB, then the above laser diode shows a reducedactive wavelength chirping Δλ of not more than 0.2 nanometers. Bycontrast, when the conventional laser diode is subjected to a directmodulation at --20%dB, then the conventional laser diode shows a reducedactive wavelength chirping Δλ of not more than 0.4 nanometers. Theactive wavelength chirping, that the above novel laser diode shows, isabout a half of the active wavelength chirping, that the aboveconventional laser diode shows. It was also confirmed that the abovenovel laser diode has a threshold current of 10 mA. The measuredefficiency of the above novel laser diode is not less than 0.2 W/A. Theoutput of the above novel laser diode is not less than 20 mW. The abovegrowth method allows that the optical guide layers have reduced internallosses and show high outputs. The above fabrication processes allows ahigh yield and a reduction in the cost of manufacturing.

A fifth embodiment according to the present invention will be describedwith reference to FIGS. 13 and 14A-14C. A novel laser diode is providedas illustrated in FIGS. 13 and 14A. The novel laser diode is formed onan n-type InP substrate 1. A surface of the n-type InP substrate 1 isformed with a grating 9. An optical guide layer 2 is provided whichextends over the grating 9. The optical guide layer 2 is made of InGaAswhich has a wavelength composition of 1.2 micrometers. The optical guidelayer 2 has a thickness of 0.1 micrometers. An active layer is providedwhich extends over the optical guide layer 2. The active layer has amultiple quantum well structure including eight quantum wells. Anoptical guide layer 4 is provided which extends over the active layer 3.The optical guide layer 4 is made of InGaAsP which has a wavelengthcomposition of 1.2 micrometers. The optical guide layer 4 has athickness of 0.1 micrometers. A p-type InP cladding layer 5 is provided,which extends over the optical guide layer 4. The p-type InP claddinglayer 5 has a thickness of 0.05 micrometers and a carrier concentrationof 5×10¹⁷ cm⁻³. A p-type InP layer 6 is provided which extends over thep-type InP cladding layer 5. The p-type InP layer 6 has a thickness of1.3 micrometers and a carrier concentration of 5×10¹⁷ cm⁻³. A p-typeInGaAsP contact layer 7 is provided which extends over the p-type InPlayer 6. The p-type InGaAsP contact layer 7 has a thickness of 0.25micrometers and a carrier concentration of 8×10¹⁸ cm⁻³. A p-typeelectrode is provided on the p-type InGaAsP contact layer 7. The abovecarrier concentration of the p-type InGaAsP contact layer 7 issufficiently high to form an ohmic contact.

The following descriptions will focus on the multiple quantum wellstructure of the active layer 3 with reference to FIGS. 14B and 14C. Themultiple quantum well structure comprises eight periods of the followinglamination structures. An electro-luminescence well layer 35 is providedin contact with a bulk region serving as a potential barrier. Apotential barrier layer 36 is provided in contact with theelectro-luminescence well layer 35. A carrier accumulation well layer 37is provided in contact with the potential barrier layer 36. Thepotential barrier layer 36 is provided in contact with the carrieraccumulation well layer 37. The electro-luminescence well layer 35 isagain provided in contact with the potential barrier layer 36. Thepotential barrier layer 36 is provided in contact with theelectro-luminescence well layer 35. The carrier accumulation well layer37 is provided in contact with the potential barrier layer 36.

Namely, each of the electro-luminescence well layers 35 is sandwichedbetween the potential barrier layers 36 as well as each of the carrieraccumulation well layers 37 is also sandwiched between the potentialbarrier layers 36 so that each of the electro-luminescence well layers35 is separated view the potential barrier layer 36 from each of thecarrier accumulation well layers 37 The electro-luminescence well layer35 comprises a +0.6%-strained InGaAs layer which has a thickness of 5.0nanometers and a wavelength composition of 1.67 micrometers. Thepotential barrier layer 36 comprises a non-strained InGaAs which has athickness of 3.0 micrometers and a wavelength composition of 1.15micrometers. The carrier accumulation well layer 37 comprises anon-strained InGaAs layer which has a thickness of 4.0 nanometers and awavelength composition of 1.40 micrometers. The electro-luminescencewell layer 35 has an energy band gap E_(g1) of 0.8.0 eV, where energyband gap is defined as a difference between a ground level of electronsin conduction band and a ground level of holes in valence band. Theenergy band gap E_(g1) of 0.8.0 eV of the electro-luminescence welllayer 35 is converted into a wavelength of 1.55 micrometers. The carrieraccumulation well layer 37 has all energy band gap E_(g2) of 1.00 eV,where energy band gap is defined as a difference between a ground levelof electrons in conduction band and a ground level of holes in valenceband. The energy band gap E_(g2) of 1.00 eV of the carrier accumulationwell layer 37 is converted into a wavelength of 1.25 micrometers.

The above electro-luminescence well layer 35 having the small energyband gap E_(g1) of 0.8.0 eV shows electro-luminescence at a wavelengthof 1.55 micrometers. On the other hand, the carrier accumulation welllayers 37 exhibit carrier accumulations without any electroluminescenceto thereby ensure that carriers accumulated in said second well layersare injected into said first well layers when said first well layers aredeficient in carriers for said electro-luminescence. The above carrieraccumulation well layer, in which carriers are accumulated for injectioninto the electro-luminescence well layers when the electro-luminescencewell layers are deficient in carriers, makes the electro-luminescencewell layers free from being deficient in carriers for theelectro-luminescence. This causes almost no variation in refractiveindex of a laser medium or a considerable reduction in variation inrefractive index of a laser medium. This suppresses an active wavelengthchirping.

It was confirmed that when the above laser diode is subjected to adirect modulation at -20%dB, then the above laser diode shows a reducedactive wavelength chirping Δλ of not more than 0.2 nanometers. Bycontrast, when the conventional laser diode is subjected to a directmodulation at -20%dB, then the conventional laser diode shows a reducedactive wavelength chirping Δλ of not more than 0.4 nanometers. Theactive wavelength chirping, that the above novel laser diode shows, isabout a half of the active wavelength chirping, that the aboveconventional laser diode shows. It was also confirmed that the abovenovel laser diode has a threshold current of 10 mA. The measuredefficiency of the above novel laser diode is not less than 0.2 W/A. Theoutput of the above novel laser diode is not less than 20 mW. The abovegrowth method allows that the optical guide layers have reduced internallosses and show high out-puts. The above fabrication processes allows ahigh yield and a reduction in the cost of manufacturing.

A third embodiment according to the present invention will be describedwith reference to FIGS. 15 and 16A-16C. A novel laser diode is providedas illustrated in FIGS. 15 and 16A. The novel laser diode is formed onan n-type InP substrate 1. A surface of the n-type InP substrate 1 isformed with a grating 9. An optical guide layer 2 is provided whichextends over the grating 9. The optical guide layer 2 is made of InGaAswhich has a wavelength composition of 1.2 micrometers. The optical guidelayer 2 has a thickness of 0.1 micrometers. An active layer is providedwhich extends over the optical guide layer 2. The active layer has amultiple quantum well structure including eight quantum wells. Anoptical guide layer 4 is provided which extends over the active layer 3.The optical guide layer 4 is made of InGaAsP which has a wavelengthcomposition of 1.2 micrometers. The optical guide layer 4 has athickness of 0.1 micrometers. A p-type InP cladding layer 5 is provided,which extends over the optical guide layer 4. The p-type InP claddinglayer 5 has a thickness of 0.05 micrometers and a carrier concentrationof 5×10¹⁷ cm⁻³. A p-type InP layer 6 is provided which extends over thep-type InP cladding layer 5. The p-type InP layer 6 has a thickness of1.3 micrometers and a carrier concentration of 5×10¹⁷ cm⁻³. A p-typeInGaAsP contact layer 7 is provided which extends over the p-type InPlayer 6. The p-type InGaAsP contact layer 7 has a thickness of 0.25micrometers and a carrier concentration of 8×10¹⁸ cm⁻³. A p-typeelectrode is provided on the p-type InGaAsP contact layer 7. The abovecarrier concentration of the p-type InGaAsP contact layer 7 issufficiently high to form an ohmic contact.

The following descriptions will focus on the multiple quantum wellstructure of the active layer 3 with reference to FIGS. 16B and 16C. Themultiple quantum well structure comprises eight periods of the followinglamination structures. A carrier accumulation layer 38 is provided incontact with a bulk region serving as a potential barrier. Anelectro-luminescence well layer 39 is provided in contact with thecarrier accumulation layer 38. A potential barrier layer 40 is providedin contact with the electro-luminescence well layer 39. The carrieraccumulation well layer 38 is provided in contact with the potentialbarrier layer 40.

Namely, each of the electro-luminescence well layers 39 is sandwichedbetween the potential barrier layer 40 and the carrier accumulationlayer 38. In other word, each of the carrier accumulation well layer 38is sandwiched between the potential barrier layer 40 and theelectro-luminescence well layer 39.

The electro-luminescence well layer 39 comprises a +0.6%-strained InGaAslayer which has a thickness of 4.5 nanometers and a wavelengthcomposition of 1.67 micrometers. The potential barrier layer 40comprises a non-strained InGaAs which has a thickness of 4.0 micrometersand a wavelength composition of 1.15 micrometers. The carrieraccumulation well layer 38 comprises a non-strained InGaAs layer whichhas a thickness of 6.0 nanometers and a wavelength composition of 1.30micrometers. The electro-luminescence well layer 39 has an energy bandgap E_(g1) of 0.8.0 eV, where energy band gap is defined as a differencebetween a ground level of electrons in conduction band and a groundlevel of holes in valence band. The energy band gap E_(g1) of 0.8.0 eVof the electro-luminescence well layer 39 is converted into a wavelengthof 1.55 micrometers. The carrier accumulation well layer 38 has anenergy band gap E_(g2) of 1.00 eV, where energy band gap is defined as adifference between a ground level of electrons in conduction band and aground level of holes in valence band. The energy band gap E_(g2) of1.00 eV of the carrier accumulation well layer 38 is converted into awavelength of 1.25 micrometers.

The above electro-luminescence well layer 39 having the small energyband gap E_(g1) of 0.8.0 eV shows electro-luminescence at a wavelengthof 1.55 micrometers. On the other hand, the carrier accumulation welllayers 38 exhibit carrier accumulations without any electro-luminescenceto thereby ensure that carriers accumulated in said second well layersare injected into said first well layers when said first well layers aredeficient in carriers for said electro-luminescence. The above carrieraccumulation well layer, in which carriers are accumulated for injectioninto the electro-luminescence well layers when the electro-luminescencewell layers are deficient in carriers, makes the electro-luminescencewell layers free from being deficient in carriers for theelectro-luminescence. This causes almost no variation in refractiveindex of a laser medium or a considerable reduction in variation inrefractive index of a laser medium. This suppresses an active wavelengthchirping.

It was confirmed that when the above laser diode is subjected to adirect modulation at -20%dB, then the above laser diode shows a reducedactive wavelength chirping Δλ of not more than 0.2 nanometers. Bycontrast, when the conventional laser diode is subjected to a directmodulation at -20%dB, then the conventional laser diode shows a reducedactive wavelength chirping Δλ of not more than 0.4 nanometers. Theactive wavelength chirping, that the above novel laser diode shows, isabout a half of the active wavelength chirping, that the aboveconventional laser diode shows. It was also confirmed that the abovenovel laser diode has a threshold current of 10 mA. The measuredefficiency of the above novel laser diode is not less than 0.2 W/A. Theoutput of the above novel laser diode is not less than 20 mW. The abovegrowth method allows that the optical guide layers have reduced internallosses and show high outputs. The above fabrication processes allows ahigh yield and a reduction in the cost of manufacturing.

A seventh embodiment according to the present invention will bedescribed with reference to FIGS. 17 and 18A-18C. A novel laser diode isprovided as illustrated in FIGS. 17 and 18A. The novel laser diode isformed on an n-type InP substrate 1. A surface of the n-type InPsubstrate 1 is formed with a grating 9. An optical guide layer 2 isprovided which extends over the grating 9. The optical guide layer 2 ismade of InGaAs which has a wavelength composition of 1.2 micrometers.The optical guide layer 2 has a thickness of 0.1 micrometers. An activelayer is provided which extends over the optical guide layer 2. Theactive layer has a multiple quantum well structure including eightquantum wells. An optical guide layer 4 is provided which extends overthe active layer 3. The optical guide layer 4 is made of InGaAsP whichhas a wavelength composition of 1.2 micrometers. The optical guide layer4 has a thickness of 0.1 micrometers. A p-type InP cladding layer 5 isprovided, which extends over the optical guide layer 4. The p-type InPcladding layer 5 has a thickness of 0.05 micrometers and a carrierconcentration of 5×10¹⁷ cm⁻³. A p-type InP layer 6 is provided whichextends over the p-type InP cladding layer 5. The p-type InP layer 6 hasa thickness of 1.3 micrometers and a carrier concentration of 5×10¹⁷cm⁻³. A p-type InGaAsP contact layer 7 is provided which extends overthe p-type InP layer 6. The p-type InGaAsP contact layer 7 has athickness of 0.25 micrometers and a carrier concentration of 8×10¹⁸cm⁻³. A p-type electrode is provided on the p-type InGaAsP contact layer7. The above carrier concentration of the p-type InGaAsP contact layer 7is sufficiently high to form an ohmic contact.

The following descriptions will focus on the multiple quantum wellstructure of the active layer 3 with reference to FIGS. 18B and 18C. Themultiple quantum well structure comprises eight periods of alternatinglaminations of electro-luminescence layers 31 and potential barrierlayers 32. A first carrier accumulation layer 41 is provided under theabove alternating laminations of the electro-luminescence layers 31 andthe potential barrier layers 32. The first carrier accumulation layer 41is separated from the potential barrier layer 32 from theelectro-luminescence layers 31. A second carrier accumulation layer 42is provided over the above alternating laminations of theelectro-luminescence layers 31 and the potential barrier layers 32. Thesecond carrier accumulation layer 42 is separated from the potentialbarrier layer 32 from the electro-luminescence layers 31.

Namely, the above alternating laminations of the electro-luminescencelayers 31 and the potential barrier layers 32 is sandwiched between thefirst and second carrier accumulation layers 41 and 42.

The electro-luminescence well layer 31 comprises a +0.6%-strained InGaAslayer which has a thickness of 5.0 nanometers and a wavelengthcomposition of 1.67 micrometers. The potential barrier layer 32comprises a non-strained InGaAs which has a thickness of 4.0 micrometersand a wavelength composition of 1.15 micrometers. The first carrieraccumulation well layer 41 has a double hetero structure, and comprisesa non-strained InGaAs layer which has a thickness of 50 nanometers and awavelength composition of 1.30 micrometers. The second carrieraccumulation well layer 42 has a double hetero structure, and comprisesa non-strained InGaAs layer which has a thickness of 50 nanometers and awavelength composition of 1.30 micrometers. The electro-luminescencewell layer 31 has an energy band gap E_(g1) of 0.8.0 eV, where energyband gap is defined as a difference between a ground level of electronsin conduction band and a ground level of holes in valence band. Theenergy band gap E_(g1) of 0.8.0 eV of the electro-luminescence welllayer 31 is converted into a wavelength of 1.55 micrometers. The firstand second carrier accumulation well layers 41 and 42 have an energyband gap which is converted into a wavelength of 1.30 micrometers.

The above electro-luminescence well layer 31 having the small energyband gap E_(g1) of 0.8.0 eV shows electro-luminescence at a wavelengthof 1.55 micrometers. On the other hand, the first and second carrieraccumulation well layers 41 and 42 exhibit carrier accumulations withoutany electro-luminescence to thereby ensure that carriers accumulated insaid second well layers are injected into said first well layers whensaid first well layers are deficient in carriers for saidelectro-luminescence. The above carrier accumulation well layers, inwhich carriers are accumulated for injection into theelectro-luminescence well layers when the electro-luminescence welllayers are deficient in carriers, makes the electro-luminescence welllayers free from being deficient in carriers for theelectro-luminescence. This causes almost no variation in refractiveindex of a laser medium or a considerable reduction in variation inrefractive index of a laser medium. This suppresses an active wavelengthchirping.

It was confirmed that when the above laser diode is subjected to adirect modulation at -20%dB, then the above laser diode shows a reducedactive wavelength chirping Δλ of not more than 0.2 nanometers. Bycontrast, when the conventional laser diode is subjected to a directmodulation at -20%dB, then the conventional laser diode shows a reducedactive wavelength chirping Δλ of not more than 0.4 nanometers. Theactive wavelength chirping, that the above novel laser diode shows, isabout a half of the active wavelength chirping, that the aboveconventional laser diode shows. It was also confined that the abovenovel laser diode has a threshold current of 10 mA. The measuredefficiency of the above novel laser diode is not less than 0.2 W/A. Theoutput of the above novel laser diode is not Less than 20 mW. The abovegrowth method allows that the optical guide layers have reduced internallosses and show high outputs. The above fabrication processes allows ahigh yield and a reduction in the cost of manufacturing.

Whereas modifications of the present invention will no doubt be apparentto a person having ordinary skill in the art, to which the inventionpertains, it is understood that embodiments as shown and described byway of illustrations are by no means intended to be considered in alimiting sense. Accordingly, it is to be intended to cover by claims allmodifications which fall within the spirit and scope of the presentinvention.

What is claimed is:
 1. An active layer structure provided in a lightemission device for emitting a light with a predetermined wavelength,said active layer structure comprising:a multiple quantum well structurecomprising alternating laminations of first well layers showingelectroluminescence and potential barrier layers, said first well layershaving a first set of energy band gaps which are uniform and correspondsto said predetermined wavelength, provided that energy band gap isdefined as a difference between a ground level of electrons inconduction band and a ground level of holes in valence band; and atleast a second well layer being provided within any of said potentialbarrier layers so that said second well layer is separated via saidpotential barrier layers from said first well layers, wherein saidsecond well layer has a second energy band gap in a range which is abovesaid first set of energy band gaps and below a set of forbidden bandwidths of said potential barrier layers, and wherein said range of saidsecond energy band gaps is defined so that said second well layerexhibits carrier accumulations and no electroluminescence to therebyensure that carriers accumulated in said second well layer are injectedinto said first well layers when said first well layers are deficient incarriers for said electroluminescence.
 2. The active layer structure asclaimed in claim 1,wherein said second well layers are provided withinevery said potential barrier layers, wherein said second well layershave a second set of energy band gaps in said range which is above saidfirst set of energy band gaps and below a set of forbidden band widthsof said potential barrier layers, and wherein said range of said secondset of energy band gaps is defined so that said second well layerexhibits carrier accumulations and no electroluminescence to therebyensure that carriers accumulated in said second well layer are injectedinto said first well layers when said first well layers are deficient incarriers for said electroluminescence.
 3. The active layer structure asclaimed in claim 1, further comprising at least a third well layer beingprovided on any interface of said first well layers and said potentialbarrier layers so that said third well layer is sandwiched between saidfirst well layer and said potential barrier layer,wherein said thirdwell layer has a third energy band gap in said range which is above saidfirst set of energy band gaps and below said set of forbidden bandwidths of said potential barrier layers, and wherein said range of saidthird energy band gap is defined so that said third well layer exhibitscarrier accumulations and no electroluminescence to thereby ensure thatcarriers accumulated in said third well layer are injected into saidfirst well layers when said first well layers are deficient in carriersfor said electroluminescence.
 4. The active layer structure as claimedin claim 3,wherein said third well layers are provided to be sandwichedby every combinations of said first well layers and said potentialbarrier layers so that every said first well layers are sandwichedbetween said third well layers and said potential barrier layers,wherein said third well layers have a third set of energy band gaps insaid range which is above said first set of energy band gaps and belowsaid set of forbidden band widths of said potential barrier layers, andwherein said range of said third set of energy band gaps is defined sothat said third well layer exhibits carrier accumulations and noelectroluminescence to thereby ensure that carriers accumulated in saidthird well layer are injected into said first well layers when saidfirst well layers are deficient in carriers for saidelectroluminescence.
 5. The active layer structure as claimed in claim 2further comprising at least a third well layer being provided on anyinterface of said first well layers and said potential barrier layers sothat said third well layer is sandwiched between said first well layerand said potential barrier layer,wherein said third well layer has athird energy band gap in said range which is above said first set ofenergy band gaps and below said set of forbidden band widths of saidpotential barrier layers, and wherein said range of said third energyband gap is defined so that said third well layer exhibits carrieraccumulations and no electroluminescence to thereby ensure that carriersaccumulated in said third well layer are injected into said first welllayers when said first well layers are deficient in carriers for saidelectroluminescence.
 6. The active layer structure as claimed in claim5,wherein said third well layers are provided to be sandwiched by everycombinations of said first well layers and said potential barrier layersso that every said first well layers are sandwiched between said thirdwell layers and said potential barrier layers, wherein said third welllayers have a third set of energy band gaps in said range which is abovesaid first set of energy band gaps and below said set of forbidden bandwidths of said potential barrier layers, and wherein said range of saidthird set of energy band gaps is defined so that said third well layerexhibits carrier accumulations and no electroluminescence to therebyensure that carriers accumulated in said third well layer are injectedinto said first well layers when said first well layers are deficient incarriers for said electroluminescence.
 7. The active layer structure asclaimed in claim 3,wherein said third well layers are provided withinevery said interfaces of said first well layers and said potentialbarrier layers so that every said first well layers are sandwiched bysaid third well layers, wherein said third well layers have a third setof energy band gaps in said range which is above said first set ofenergy band gaps and below said set of forbidden band widths of saidpotential barrier layers, and wherein said range of said second set ofenergy band gaps is defined so that said third well layer exhibitscarrier accumulations and no electroluminescence to thereby ensure thatcarriers accumulated in said third well layer are injected into saidfirst well layers when said first well layers are deficient in carriersfor said electroluminescence.
 8. The active layer structure as claimedin claim 5,wherein said third well layers are provided within every saidinterfaces of said first well layers and said potential barrier layersso that every said first well layers are sandwiched by said third welllayers, wherein said third well layers have a third set of energy bandgaps in said range which is above said first set of energy band gaps andbelow said set of forbidden band widths of said potential barrierlayers, and wherein said range of said second set of energy band gaps isdefined so that said third well layer exhibits carrier accumulations andno electroluminescence to thereby ensure that carriers accumulated insaid third well layer are injected into said first well layers when saidfirst well layers are deficient in carriers for saidelectroluminescence.
 9. The active layer structure as claimed in claim1, further comprising at least a fourth well layer being provided at anyside of said multiple quantum well structure so that said fourth welllayer is separated via said potential barrier layers from said firstwell layers,wherein said fourth well layer has a fourth energy band gapin said range which is above said first set of energy band gaps andbelow said set of forbidden band widths of said potential barrierlayers, and wherein said range of said second energy band gap is definedso that said fourth well layer exhibits carrier accumulations and noelectroluminescence to thereby ensure that carriers accumulated in saidfourth well layer are injected into said first well layers when saidfirst well layers are deficient in carriers for saidelectroluminescence.
 10. The active layer structure as claimed in claim9, wherein said fourth well layers are provided at opposite sides ofsaid multiple quantum well structure so that said multiple quantum wellstructure is positioned between said fourth well layers.
 11. The activelayer structure as claimed in claim 2, further comprising at least afourth well layer being provided at any side of said multiple quantumwell structure so that said fourth well layer is separated via saidpotential barrier layers from said first well layers,wherein said fourthwell layer has a fourth energy band gap in said range which is abovesaid first set of energy band gaps and below said set of forbidden bandwidths of said potential barrier layers, and wherein said range of saidsecond energy band gap is defined so that said fourth well layerexhibits carrier accumulations and no electroluminescence to therebyensure that carriers accumulated in said fourth well layer are injectedinto said first well layers when said first well layers are deficient incarriers for said electroluminescence.
 12. The active layer structure asclaimed in claim 11, wherein said fourth well layers are provided atopposite sides of said multiple quantum well structure so that saidmultiple quantum well structure is positioned between said fourth welllayers.
 13. The active layer structure as claimed in claim 1,whereinsaid first and second well layers and said potential barrier layers varyin forbidden band width and in wavelength composition as well as inthickness, so that every said first well layers have a uniform energyband gap which corresponds to said predetermined wavelength, and so thatsaid second energy band gap is in said range which is above said firstset of energy band gaps and below said set of forbidden band widths ofsaid potential barrier layers, where said range of said second energyband gap is defined so that said second well layer exhibits carrieraccumulations and no electro-luminescence to thereby ensure thatcarriers accumulated in said second well layer is injected into saidfirst well layers when said first well layers are deficient in carriersfor said electroluminescence.
 14. The active layer structure as claimedin claim 2,wherein said first and second well layers and said potentialbarrier layers vary in forbidden band width and in wavelengthcomposition as well as in thickness, so that every said first welllayers have a uniform energy band gap which corresponds to saidpredetermined wavelength, and so that said second set of energy bandgaps is in said range which is above said first set of energy band gapsand below said set of forbidden band widths of said potential barrierlayers, where said range of said second set of energy band gaps isdefined so that said second well layers exhibit carrier accumulationsand no electro-luminescence to thereby ensure that carriers accumulatedin said second well layer is injected into said first well layers whensaid first well layers are deficient in carriers for saidelectroluminescence.
 15. The active layer structure as claimed in claim1,wherein said first well layers have a uniform forbidden band width anda uniform wavelength composition as well as a uniform thickness, andwherein said potential barrier layers also have a uniform forbidden bandwidth and a uniform wavelength composition as well as a uniformthickness.
 16. The active layer structure as claimed in claim 2,whereinsaid first well layers have a uniform forbidden band width and a uniformwavelength composition as well as a uniform thickness, wherein saidpotential barrier layers also have a uniform forbidden band width and auniform wavelength composition as well as a uniform thickness, andwherein said second well layers have a uniform forbidden band width anda uniform wavelength composition as well as a uniform thickness.
 17. Theactive layer structure as claimed in claim 1,wherein said first welllayers have a wavelength composition of 1.67 micrometers and a thicknessof 5.0 nanometers as well as a first energy band gap of 0.8.0 eV,wherein said potential barrier layers have a wavelength composition of1.15 micrometers and of a thickness 3 micrometers, wherein said secondwell layer has a wavelength composition of 1.40 micrometers and athickness of 4.0 nanometers as well as a second energy band gap of 1.00eV.
 18. The active layer structure as claimed in claim 17,wherein saidfirst well layers are made of +0.6%-strained InGaAs layers, wherein saidpotential barrier layers are made of non-strained InGaAs layers, andwherein said second well layer is made of a non-strained InGaAs layer.19. A semiconductor laser device including an active layer structurewhich is provided on an optical guide layer provided on a surface,having a grating structure, of a semiconductor substrate, said activelayer structure comprising:a multiple quantum well structure comprisingalternating laminations of first well layers showing electroluminescenceand potential barrier layers, said first well layers having a first setof energy band gaps which are uniform and corresponds to saidpredetermined wavelength, provided that energy band gap is defined as adifference between a ground level of electrons in conduction band and aground level of holes in valence band; and at least a second well layerbeing provided within any of said potential barrier layers so that saidsecond well layer is separated via said potential barrier layers fromsaid first well layers, wherein said second well layer has a secondenergy band gap in a range which is above said first set of energy bandgaps and below a set of forbidden band widths of said potential barrierlayers, and wherein said range of said second energy band gaps isdefined so that said second well layer exhibits carrier accumulationsand no electroluminescence to thereby ensure that carriers accumulatedin said second well layer are injected into said first well layers whensaid first well layers are deficient in carriers for saidelectroluminescence.
 20. The semiconductor laser device as claimed inclaim 19,wherein said second well layers are provided within every saidpotential barrier layers, wherein said second well layers have a secondset of energy band gaps in said range which is above said first set ofenergy band gaps and below a set of forbidden band widths of saidpotential barrier layers, and wherein said range of said second set ofenergy band gaps is defined so that said second well layer exhibitscarrier accumulations and no electroluminescence to thereby ensure thatcarriers accumulated in said second well layer are injected into saidfirst well layers when said first well layers are deficient in carriersfor said electroluminescence.
 21. The semiconductor laser device asclaimed in claim 19,wherein said first and second well layers and saidpotential barrier layers vary in forbidden band width and in wavelengthcomposition as well as in thickness, so that every said first welllayers have a uniform energy band gap which corresponds to saidpredetermined wavelength, and so that said second energy band gap is insaid range which is above said first set of energy band gaps and belowsaid set of forbidden band widths of said potential barrier layers,where said range of said second energy band gap is defined so that saidsecond well layer exhibits carrier accumulations and noelectro-luminescence to thereby ensure that carriers accumulated in saidsecond well layer is injected into said first well layers when saidfirst well layers are deficient in carriers for saidelectroluminescence.
 22. The semiconductor laser device as claimed inclaim 20,wherein said first and second well layers and said potentialbarrier layers vary in forbidden band width and in wavelengthcomposition as well as in thickness, so that every said first welllayers have a uniform energy band gap which corresponds to saidpredetermined wavelength, and so that said second set of energy bandgaps is in said range which is above said first set of energy band gapsand below said set of forbidden band widths of said potential barrierlayers, where said range of said second set of energy band gaps isdefined so that said second well layers exhibit carrier accumulationsand no electro-luminescence to thereby ensure that carriers accumulatedin said second well layer is injected into said first well layers whensaid first well layers are deficient in carriers for saidelectroluminescence.
 23. The semiconductor laser device as claimed inclaim 19,wherein said first well layers have a uniform forbidden bandwidth and a uniform wavelength composition as well as a uniformthickness, and wherein said potential barrier layers also have a uniformforbidden band width and a uniform wavelength composition as well as auniform thickness.
 24. The semiconductor laser device as claimed inclaim 20,wherein said first well layers have a uniform forbidden bandwidth and a uniform wavelength composition as well as a uniformthickness, wherein said potential barrier layers also have a uniformforbidden band width and a uniform wavelength composition as well as auniform thickness, and wherein said second well layers have a uniformforbidden band width and a uniform wavelength composition as well as auniform thickness.
 25. The semiconductor laser device as claimed inclaim 19,wherein said first well layers have a wavelength composition of1.67 micrometers and a thickness of 5.0 nanometers as well as a firstenergy band gap of 0.8.0 eV, wherein said potential barrier layers havea wavelength composition of 1.15 micrometers and of a thickness 3micrometers, wherein said second well layer has a wavelength compositionof 1.40 micrometers and a thickness of 4.0 nanometers as well as asecond energy band gap of 1.00 eV.
 26. The semiconductor laser device asclaimed in claim 25,wherein said first well layers are made of+0.6%-strained InGaAs layers, wherein said potential barrier layers aremade of non-strained InGaAs layers, and wherein said second well layeris made of a non-strained InGaAs layer.
 27. An active layer structureprovided in a light emission device for emitting a light with apredetermined wavelength, said active layer structure comprising:amultiple quantum well structure comprising alternating laminations offirst well layers showing electroluminescence and potential barrierlayers, said first well layers having a first set of energy band gapswhich are uniform and corresponds to said predetermined wavelength,provided that energy band gap is defined as a difference between aground level of electrons in conduction band and a ground level of holesin valence band; and at least a second well layer being provided on anyinterface of said first well layers and said potential barrier layers sothat said second well layer is sandwiched between said first well layerand said potential barrier layer, wherein said second well layer has asecond energy band gap in a range which is above said first set ofenergy band gaps and below a set of forbidden band widths of saidpotential barrier layers, and wherein said range of said second energyband gap is defined so that said second well layer exhibits carrieraccumulations and no electroluminescence to thereby ensure that carriersaccumulated in said second well layer are injected into said first welllayers when said first well layers are deficient in carriers for saidelectroluminescence.
 28. The active layer structure as claimed in claim27,wherein said second well layers are provided to be sandwiched byevery combinations of said first well layers and said potential barrierlayers so that every said first well layers are sandwiched between saidsecond well layers and said potential barrier layers, wherein saidsecond well layers have a second set of energy band gaps in said rangewhich is above said first set of energy band gaps and below said set offorbidden band widths of said potential barrier layers, and wherein saidrange of said second set of energy band gaps is defined so that saidsecond well layer exhibits carrier accumulations and noelectroluminescence to thereby ensure that carriers accumulated in saidsecond well layer are injected into said first well layers when saidfirst well layers are deficient in carriers for saidelectroluminescence.
 29. The active layer structure as claimed in claim27,wherein said second well layers are provided within every saidinterfaces of said first well layers and said potential barrier layersso that every said first well layers are sandwiched by said second welllayers, wherein said second well layers have a second set of energy bandgaps in said range which is above said first set of energy band gaps andbelow said set of forbidden band widths of said potential barrierlayers, and wherein said range of said second set of energy band gaps isdefined so that said second well layer exhibits carrier accumulationsand no electroluminescence to thereby ensure that carriers accumulatedin said second well layer are injected into said first well layers whensaid first well layers are deficient in carriers for saidelectroluminescence.
 30. The active layer structure as claimed in claim27, further comprising at least a third well layer being provided withinany of said potential barrier layers so that said third well layer isseparated via said potential barrier layers from said first welllayers,wherein said third well layer has a third energy band gap in saidrange which is above said first set of energy band gaps and below saidset of forbidden band widths of said potential barrier layers, andwherein said range of said third energy band gap is defined so that saidthird well layer exhibits carrier accumulations and noelectroluminescence to thereby ensure that carriers accumulated in saidthird well layer are injected into said first well layers when saidfirst well layers are deficient in carriers for saidelectroluminescence.
 31. The active layer structure as claimed in claim30,wherein said third well layers are provided within every saidpotential barrier layers, wherein said third well layers have a thirdset of energy band gaps in said range which is above said first set ofenergy band gaps and below said set of forbidden band widths of saidpotential barrier layers, and wherein said range of said third set ofenergy band gaps is defined so that said third well layer exhibitscarrier accumulations and no electroluminescence to thereby ensure thatcarriers accumulated in said third well layer are injected into saidfirst well layers when said fist well layers are deficient in carriersfor said electroluminescence.
 32. The active layer structure as claimedin claim 28, further comprising at least a third well layer beingprovided within any of said potential barrier layers so that said thirdwell layer is separated via said potential barrier layers from saidfirst well layers,wherein said third well layer has a third energy bandgap in said range which is above said first set of energy band gaps andbelow said set of forbidden band widths of said potential barrierlayers, and wherein said range of said third energy band gap is definedso that said third well layer exhibits carrier accumulations and noelectroluminescence to thereby ensure that carriers accumulated in saidthird well layer are injected into said first well layers when saidfirst well layers are deficient in carriers for saidelectroluminescence.
 33. The active layer structure as claimed in claim32,wherein said third well layers are provided within every saidpotential barrier layers, wherein said third well layers have a thirdset of energy band gaps in said range which is above said first set ofenergy band gaps and below said set of forbidden band widths of saidpotential barrier layers, and wherein said range of said third set ofenergy band gaps is defined so that said third well layer exhibitscarrier accumulations and no electroluminescence to thereby ensure thatcarriers accumulated in said third well layer are injected into saidfirst well layers when said first well layers are deficient in carriersfor said electroluminescence.
 34. The active layer structure as claimedin claim 27, further comprising at least a third well layer beingprovided within any of said potential barrier layers so that said thirdwell layer is separated via said potential barrier layers from saidfirst well layers,wherein said third well layer has a third energy bandgap in said range which is above said first set of energy band gaps andbelow said set of forbidden band widths of said potential barrierlayers, and wherein said range of said third energy band gap is definedso that said third well layer exhibits carrier accumulations and noelectroluminescence to thereby ensure that carriers accumulated in saidthird well layer are injected into said first well layers when saidfirst well layers are deficient in carriers for saidelectroluminescence.
 35. The active layer structure as claimed in claim34,wherein said third well layers are provided within every saidpotential barrier layers, wherein said third well layers have a thirdset of energy band gaps in said range which is above said first set ofenergy band gaps and below said set of forbidden band widths of saidpotential barrier layers, and wherein said range of said third set ofenergy band gaps is defined so that said third well layer exhibitscarrier accumulations and no electroluminescence to thereby ensure thatcarriers accumulated in said third well layer are injected into saidfirst well layers when said first well layers are deficient in carriersfor said electroluminescence.
 36. The active layer structure as claimedin claim 27, further comprising at least a fourth well layer beingprovided at any side of said multiple quantum well structure so thatsaid fourth well layer is separated via said potential barrier layersfrom said first well layers,wherein said fourth well layer has a fourthenergy band gap in said range which is above said first set of energyband gaps and below said set of forbidden band widths of said potentialbarrier layers, and wherein said range of said second energy band gap isdefined so that said fourth well layer exhibits carrier accumulationsand no electroluminescence to thereby ensure that carriers accumulatedin said fourth well layer are injected into said first well layers whensaid first well layers are deficient in carriers for saidelectroluminescence.
 37. The active layer structure as claimed in claim36, wherein said fourth well layers are provided at opposite sides ofsaid multiple quantum well structure so that said multiple quantum wellstructure is positioned between said fourth well layers.
 38. The activelayer structure as claimed in claim 28, further comprising at least afourth well layer being provided at any side of said multiple quantumwell structure so that said fourth well layer is separated via saidpotential barrier layers from said first well layers,wherein said fourthwell layer has a fourth energy band gap in said range which is abovesaid first set of energy band gaps and below said set of forbidden bandwidths of said potential barrier layers, and wherein said range of saidsecond energy band gap is defined so that said fourth well layerexhibits carrier accumulations and no electroluminescence to therebyensure that carriers accumulated in said fourth well layer are injectedinto said first well layers when said first well layers are deficient incarriers for said electroluminescence.
 39. The active layer structure asclaimed in claim 38, wherein said fourth well layers are provided atopposite sides of said multiple quantum well structure so that saidmultiple quantum well structure is positioned between said fourth welllayers.
 40. The active layer structure as claimed in claim 27,whereinsaid first and second well layers and said potential barrier layers varyin forbidden band width and in wavelength composition as well as inthickness, so that every said first well layers have a uniform energyband gap which corresponds to said predetermined wavelength, and so thatsaid second energy band gap is in said range which is above said firstset of energy band gaps and below said set of forbidden band widths ofsaid potential barrier layers, where said range of said second energyband gap is defined so that said second well layer exhibits carrieraccumulations and no electro-luminescence to thereby ensure thatcarriers accumulated in said second well layer is injected into saidfirst well layers when said first well layers are deficient in carriersfor said electroluminescence.
 41. The active layer structure as claimedin claim 28,wherein said first and second well layers and said potentialbarrier layers vary in forbidden band width and in wavelengthcomposition as well as in thickness, so that every said first welllayers have a uniform energy band gap which corresponds to saidpredetermined wavelength, and so that said second set of energy bandgaps is in said range which is above said first set of energy band gapsand below said set of forbidden band widths of said potential barrierlayers, where said range of said second set of energy band gaps isdefined so that said second well layers exhibit carrier accumulationsand no electro-luminescence to thereby ensure that carriers accumulatedin said second well layer is injected into said first well layers whensaid first well layers are deficient in carriers for saidelectroluminescence.
 42. The active layer structure as claimed in claim27,wherein said first well layers have a uniform forbidden band widthand a uniform wavelength composition as well as a uniform thickness, andwherein said potential barrier layers also have a uniform forbidden bandwidth and a uniform wavelength composition as well as a uniformthickness.
 43. The active layer structure as claimed in claim 28,whereinsaid first well layers have a uniform forbidden band width and a uniformwavelength composition as well as a uniform thickness, wherein saidpotential barrier layers also have a uniform forbidden band width and auniform wavelength composition as well as a uniform thickness, andwherein said second well layers have a uniform forbidden band width anda uniform wavelength composition as well as a uniform thickness.
 44. Theactive layer structure as claimed in claim 27,wherein said first welllayers have a wavelength composition of 1.67 micrometers and a thicknessof 4.5 nanometers as well as a first energy band gap of 0.8.0 eV,wherein said potential barrier layers have a wavelength composition of1.15 micrometers and of a thickness 4 micrometers, wherein said secondwell layer has a wavelength composition of 1.30 micrometers and athickness of 6.0 nanometers as well as a second energy band gap of 1.00eV.
 45. The active layer structure as claimed in claim 44,wherein saidfirst well layers are made of +0.6%-strained InGaAs layers, wherein saidpotential barrier layers are made of non-strained InGaAs layers, andwherein said second well layer is made of a non-strained InGaAs layer.46. A semiconductor laser device including an active layer structurewhich is provided on an optical guide layer provided on a surface,having a grating structure, of a semiconductor substrate, said activelayer structure comprising:a multiple quantum well structure comprisingalternating laminations of first well layers showing electroluminescenceand potential barrier layers, said first well layers having a first setof energy band gaps which are uniform and corresponds to saidpredetermined wavelength, provided that energy band gap is defined as adifference between a ground level of electrons in conduction band and aground level of holes in valence band; and at least a second well layerbeing provided on any interface of said first well layers and saidpotential barrier layers so that said second well layer is sandwichedbetween said first well layer and said potential barrier layer, whereinsaid second well layer has a second energy band gap in a range which isabove said first set of energy band gaps and below a set of forbiddenband widths of said potential barrier layers, and wherein said range ofsaid second energy band gap is defined so that said second well layerexhibits carrier accumulations and no electroluminescence to therebyensure that carriers accumulated in said second well layer are injectedinto said first well layers when said first well layers are deficient incarriers for said electroluminescence.
 47. The semiconductor laserdevice as claimed in claim 46,wherein said second well layers areprovided to be sandwiched by every combinations of said first welllayers and said potential barrier layers so that every said first welllayers are sandwiched between said second well layers and said potentialbarrier layers, wherein said second well layers have a second set ofenergy band gaps in said range which is above said first set of energyband gaps and below said set of forbidden band widths of said potentialbarrier layers, and wherein said range of said second set of energy bandgaps is defined so that said second well layer exhibits carrieraccumulations and no electroluminescence to thereby ensure that carriersaccumulated in said second well layer are injected into said first welllayers when said first well layers are deficient in carriers for saidelectroluminescence.
 48. The semiconductor laser device as claimed inclaim 46,wherein said second well layers are provided within every saidinterfaces of said first well layers and said potential barrier layersso that every said first well layers are sandwiched by said second welllayers, wherein said second well layers have a second set of energy bandgaps in said range which is above said first set of energy band gaps andbelow said set of forbidden band widths of said potential barrierlayers, and wherein said range of said second set of energy band gaps isdefined so that said second well layer exhibits carrier accumulationsand no electroluminescence to thereby ensure that carriers accumulatedin said second well layer are injected into said first well layers whensaid first well layers are deficient in carriers for saidelectroluminescence.
 49. The semiconductor laser device as claimed inclaim 46,wherein said first and second well layers and said potentialbarrier layers vary in forbidden band width and in wavelengthcomposition as well as in thickness, so that every said first welllayers have a uniform energy band gap which corresponds to saidpredetermined wavelength, and so that said second energy band gap is insaid range which is above said first set of energy band gaps and belowsaid set of forbidden band widths of said potential barrier layers,where said range of said second energy band gap is defined so that saidsecond well layer exhibits carrier accumulations and noelectro-luminescence to thereby ensure that carriers accumulated in saidsecond well layer is injected into said first well layers when saidfirst well layers are deficient in carriers for saidelectroluminescence.
 50. The semiconductor laser device as claimed inclaim 47,wherein said first and second well layers and said potentialbarrier layers vary in forbidden band width and in wavelengthcomposition as well as in thickness, so that every said first welllayers have a uniform energy band gap which corresponds to saidpredetermined wavelength, and so that said second set of energy bandgaps is in said range which is above said first set of energy band gapsand below said set of forbidden band widths of said potential barrierlayers, where said range of said second set of energy band gaps isdefined so that said second well layers exhibit carrier accumulationsand no electro-luminescence to thereby ensure that carriers accumulatedin said second well layer is injected into said first well layers whensaid first well layers are deficient in carriers for saidelectroluminescence.
 51. The semiconductor laser device as claimed inclaim 46,wherein said first well layers have a uniform forbidden bandwidth and a uniform wavelength composition as well as a uniformthickness, and wherein said potential barrier layers also have a uniformforbidden band width and a uniform wavelength composition as well as auniform thickness.
 52. The semiconductor laser device as claimed inclaim 47,wherein said first well layers have a uniform forbidden bandwidth and a uniform wavelength composition as well as a uniformthickness, wherein said potential barrier layers also have a uniformforbidden band width and a uniform wavelength composition as well as auniform thickness, and wherein said second well layers have a uniformforbidden band width and a uniform wavelength composition as well as auniform thickness.
 53. The semiconductor laser device as claimed inclaim 46,wherein said first well layers have a wavelength composition of1.67 micrometers and a thickness of 4.5 nanometers as well as a firstenergy band gap of 0.8.0 eV, wherein said potential barrier layers havea wavelength composition of 1.15 micrometers and of a thickness 4micrometers, wherein said second well layer has a wavelength compositionof 1.30 micrometers and a thickness of 6.0 nanometers as well as asecond energy band gap of 1.00 eV.
 54. The semiconductor laser device asclaimed in claim 53,wherein said first well layers are made of+0.6%-strained InGaAs layers, wherein said potential barrier layers aremade of non-strained InGaAs layers, and wherein said second well layeris made of a non-strained InGaAs layer.
 55. An active layer structureprovided in a light emission device for emitting a light with apredetermined wavelength, said active layer structure comprising:amultiple quantum well structure comprising alternating laminations offirst well layers showing electroluminescence and potential barrierlayers, said first well layers having a first set of energy band gapswhich are uniform and corresponds to said predetermined wavelength,provided that energy band gap is defined as a difference between aground level of electrons in conduction band and a ground level of holesin valence band; and at least a second well layer being provided at anyside of said multiple quantum well structure so that said second welllayer is separated via said potential barrier layers from said firstwell layers, wherein said second well layer has a second energy band gapin a range which is above said first set of energy band gaps and below aset of forbidden band widths of said potential barrier layers, andwherein said range of said second energy band gap is defined so thatsaid second well layer exhibits carrier accumulations and noelectroluminescence to thereby ensure that carriers accumulated in saidsecond well layer are injected into said first well layers when saidfirst well layers are deficient in carriers for saidelectroluminescence.
 56. The active layer structure as claimed in claim55, wherein said second well layers are provided at opposite sides ofsaid multiple quantum well structure so that said multiple quantum wellstructure is positioned between said second well layers.
 57. The activelayer structure as claimed in claim 55, further comprising at least athird well layer being provided within any of said potential barrierlayers so that said third well layer is separated via said potentialbarrier layers from said first well layers,wherein said third well layerhas a second energy band gap in a range which is above said first set ofenergy band gaps and below said set of forbidden band widths of saidpotential barrier layers, and wherein said range of said third energyband gaps is defined so that said third well layer exhibits carrieraccumulations and no electroluminescence to thereby ensure that carriersaccumulated in said third well layer are injected into said first welllayers when said first well layers are deficient in carriers for saidelectroluminescence.
 58. The active layer structure as claimed in claim57,wherein said third well layers are provided within every saidpotential barrier layers, wherein said third well layers have a secondset of energy band gaps in said range which is above said first set ofenergy band gaps and below said set of forbidden band widths of saidpotential barrier layers, and wherein said range of said third set ofenergy band gaps is defined so that said third well layer exhibitscarrier accumulations and no electroluminescence to thereby ensure thatcarriers accumulated in said third well layer are injected into saidfirst well layers when said first well layers are deficient in carriersfor said electroluminescence.
 59. The active layer structure as claimedin claim 56, further comprising at least a third well layer beingprovided within any of said potential barrier layers so that said thirdwell layer is separated via said potential barrier layers from saidfirst well layers,wherein said third well layer has a second energy bandgap in a range which is above said first set of energy band gaps andbelow said set of forbidden band widths of said potential barrierlayers, and wherein said range of said third energy band gaps is definedso that said third well layer exhibits carrier accumulations and noelectroluminescence to thereby ensure that carriers accumulated in saidthird well layer are injected into said first well layers when saidfirst well layers are deficient in carriers for saidelectroluminescence.
 60. The active layer structure as claimed in claim59,wherein said third well layers are provided within every saidpotential barrier layers, wherein said third well layers have a secondset of energy band gaps in said range which is above said first set ofenergy band gaps and below said set of forbidden band widths of saidpotential barrier layers, and wherein said range of said third set ofenergy band gaps is defined so that said third well layer exhibitscarrier accumulations and no electroluminescence to thereby ensure thatcarriers accumulated in said third well layer are injected into saidfirst well layers when said first well layers are deficient in carriersfor said electroluminescence.
 61. The active layer structure as claimedin claim 55, further comprising at least a fourth well layer beingprovided on any interface of said first well layers and said potentialbarrier layers so that said fourth well layer is sandwiched between saidfirst well layer and said potential barrier layer,wherein said fourthwell layer has a fourth energy band gap in said range which is abovesaid first set of energy band gaps and below said set of forbidden bandwidths of said potential barrier layers, and wherein said range of saidfourth energy band gap is defined so that said fourth well layerexhibits carrier accumulations and no electroluminescence to therebyensure that carriers accumulated in said fourth well layer are injectedinto said first well layers when said first well layers are deficient incarriers for said electroluminescence.
 62. The active layer structure asclaimed in claim 61,wherein said fourth well layers are provided to besandwiched by every combinations of said first well layers and saidpotential barrier layers so that every said first well layers aresandwiched between said fourth well layers and said potential barrierlayers, wherein said fourth well layers have a fourth set of energy bandgaps in said range which is above said first set of energy band gaps andbelow said set of forbidden band widths of said potential barrierlayers, and wherein said range of said fourth set of energy band gaps isdefined so that said fourth well layer exhibits carrier accumulationsand no electroluminescence to thereby ensure that carriers accumulatedin said fourth well layer are injected into said first well layers whensaid first well layers are deficient in carriers for saidelectroluminescence.
 63. The active layer structure as claimed in claim61,wherein said fourth well layers are provided within every saidinterfaces of said first well layers and said potential barrier layersso that every said first well layers are sandwiched by said fourth welllayers, wherein said fourth well layers have a fourth set of energy bandgaps in said range which is above said first set of energy band gaps andbelow said set of forbidden band widths of said potential barrierlayers, and wherein said range of said fourth set of energy band gaps isdefined so that said fourth well layer exhibits carrier accumulationsand no electroluminescence to thereby ensure that carriers accumulatedin said fourth well layer are injected into said first well layers whensaid first well layers are deficient in carriers for saidelectroluminescence.
 64. The active layer structure as claimed in claim56, further comprising at least a fourth well layer being provided onany interface of said first well layers and said potential barrierlayers so that said fourth well layer is sandwiched between said firstwell layer and said potential barrier layer,wherein said fourth welllayer has a fourth energy band gap in said range which is above saidfirst set of energy band gaps and below said set of forbidden bandwidths of said potential barrier layers, and wherein said range of saidfourth energy band gap is defined so that said fourth well layerexhibits carrier accumulations and no electroluminescence to therebyensure that carriers accumulated in said fourth well layer are injectedinto said first well layers when said first well layers are. deficientin carriers for said electroluminescence.
 65. The active layer structureas claimed in claim 64,wherein said fourth well layers are provided tobe sandwiched by every combinations of said first well layers and saidpotential barrier layers so that every said first well layers aresandwiched between said fourth well layers and said potential barrierlayers, wherein said fourth well layers have a fourth set of energy bandgaps in said range which is above said first set of energy band gaps andbelow said set of forbidden band widths of said potential barrierlayers, and wherein said range of said fourth set of energy band gaps isdefined so that said fourth well layer exhibits carrier accumulationsand no electroluminescence to thereby ensure that carriers accumulatedin said fourth well layer are injected into said first well layers whensaid first well layers are deficient in carriers for saidelectroluminescence.
 66. The active layer structure as claimed in claim64,wherein said fourth well layers are provided within every saidinterfaces of said first well layers and said potential barrier layersso that every said first well layers are sandwiched by said fourth welllayers, wherein said fourth well layers have a fourth set of energy bandgaps in said range which is above said first set of energy band gaps andbelow said set of forbidden band widths of said potential barrierlayers, and wherein said range of said fourth set of energy band gaps isdefined so that said fourth well layer exhibits carrier accumulationsand no electroluminescence to thereby ensure that carriers accumulatedin said fourth well layer are injected into said first well layers whensaid first well layers are deficient in carriers for saidelectroluminescence.
 67. The active layer structure as claimed in claim55,wherein said first and second well layers and said potential barrierlayers vary in forbidden band width and in wavelength composition aswell as in thickness, so that every said first well layers have auniform energy band gap which corresponds to said predeterminedwavelength, and so that said second energy band gap is in said rangewhich is above said first set of energy band gaps and below said set offorbidden band widths of said potential barrier layers, where said rangeof said second energy band gap is defined so that said second well layerexhibits carrier accumulations and no electro-luminescence to therebyensure that carriers accumulated in said second well layer is injectedinto said first well layers when said first well layers are deficient incarries for said electroluminescence.
 68. The active layer structure asclaimed in claim 55,wherein said first well layers have a uniformforbidden band width and a uniform wavelength composition as well as auniform thickness, and wherein said potential barrier layers also have auniform forbidden band width and a uniform wavelength composition aswell as a uniform thickness.
 69. The active layer structure as claimedin claim 68,wherein said first well layers have a wavelength compositionof 1.67 micrometers and a thickness of 5.0 nanometers as well as a firstenergy band gap of 0.8.0 eV, wherein said potential barrier layers havea wavelength composition of 1.15 micrometers and of a thickness 4micrometers, and wherein said second well layer has a wavelengthcomposition of 1.30 micrometers and a thickness of 50 nanometers as wellas a second energy band gap which corresponds to said wavelengthcomposition of 1.30 micrometers.
 70. The active layer structure asclaimed in claim 69,wherein said first well layers are made of+0.6%-strained InGaAs layers, wherein said potential barrier layers aremade of non-strained InGaAs layers, and wherein said second well layeris made of a non-strained InGaAs layer.
 71. A semiconductor laser deviceincluding an active layer structure which is provided on an opticalguide layer provided on a surface, having a grating structure, of asemiconductor substrate, said active layer structure comprising:amultiple quantum well structure comprising alternating laminations offirst well layers showing electroluminescence and potential barrierlayers, said first well layers having a first set of energy band gapswhich are uniform and corresponds to said predetermined wavelength,provided that energy band gap is defined as a difference between aground level of electrons in conduction band and a ground level of holesin valence band; and at least a second well layer being provided at anyside of said multiple quantum well structure so that said second welllayer is separated via said potential barrier layers from said firstwell layers, wherein said second well layer has a second energy band gapin a range which is above said first set of energy band gaps and below asets of forbidden band widths of said potential barrier layers, andwherein said range of said second energy band gap is defined so thatsaid second well layer exhibits carrier accumulations and noelectroluminescence to thereby ensure that carriers accumulated in saidsecond well layer are injected into said first well layers when saidfirst well layers are deficient in carriers for saidelectroluminescence.
 72. The semiconductor laser device as claimed inclaim 71, wherein said second well layers are provided at opposite sidesof said multiple quantum well structure so that said multiple quantumwell structure is positioned between said second well layers.
 73. Thesemiconductor laser device as claimed in claim 71,wherein said first andsecond well layers and said potential barrier layers vary in forbiddenband width and in wavelength composition as well as in thickness, sothat every said first well layers have a uniform energy band gap whichcorresponds to said predetermined wavelength, and so that said secondenergy band gap is in said range which is above said first set of energyband gaps and below said set of forbidden band widths of said potentialbarrier layers, where said range of said second energy band gap isdefined so that said second well layer exhibits carrier accumulationsand no electro-luminescence to thereby ensure that carriers accumulatedin said second well layer is injected into said first well layers whensaid first well layers are deficient in carriers for saidelectroluminescence.
 74. The semiconductor laser device as claimed inclaim 71,wherein said first well layers have a uniform forbidden bandwidth and a uniform wavelength composition as well as a uniformthickness, and wherein said potential barrier layers also have a uniformforbidden band width and a uniform wavelength composition as well as auniform thickness.
 75. The semiconductor laser device as claimed inclaim 74,wherein said first well layers have a wavelength composition of1.67 micrometers and a thickness of 5.0 nanometers as well as a firstenergy band gap of 0.8.0 eV, wherein said potential barrier layers havea wavelength composition of 1.15 micrometers and of a thickness 4micrometers, and wherein said second well layer has a wavelengthcomposition of 1.30 micrometers and a thickness of 50 nanometers as wellas a second energy band gap which corresponds to said wavelengthcomposition of 1.30 micrometers.
 76. The semiconductor laser device asclaimed in claim 75,wherein said first well layers are made of+0.6%-strained InGaAs layers, wherein said potential barrier layers aremade of non-strained InGaAs layers, and wherein said second well layeris made of a non-strained InGaAs layer.